Morning and evening adhkar are a set of dhikr and short verses said at two fixed points of the day. They are collected in Hisn al-Muslim (the Fortress of the Muslim) by Sa'id al-Qahtani, and each one carries its Arabic text, a meaning, and a note on where it was recorded.
The morning set is usually said after Fajr until the sun has risen, and the evening set from the afternoon prayer until sunset.
If the time passes and you remember later, say them then — the set belongs to the day, not to a minute.
If you are starting out, these are the shortest and easiest to remember: Ayat al-Kursi once, then the last three surahs three times each.
اللَّهُ لَا إِلَٰهَ إِلَّا هُوَ الْحَيُّ الْقَيُّومُ ۚ لَا تَأْخُذُهُ سِنَةٌ وَلَا نَوْمٌ ۚ لَّهُ مَا فِي السَّمَاوَاتِ وَمَا فِي الْأَرْضِ ۗ مَن ذَا الَّذِي يَشْفَعُ عِندَهُ إِلَّا بِإِذْنِهِ ۚ يَعْلَمُ مَا بَيْنَ أَيْدِيهِمْ وَمَا خَلْفَهُمْ ۖ وَلَا يُحِيطُونَ بِشَيْءٍ مِّنْ عِلْمِهِ إِلَّا بِمَا شَاءَ ۚ وَسِعَ كُرْسِيُّهُ السَّمَاوَاتِ وَالْأَرْضَ ۖ وَلَا يَئُودُهُ حِفْظُهُمَا ۚ وَهُوَ الْعَلِيُّ الْعَظِيمُ
Allah — there is no deity except Him, the Ever-Living, the Sustainer of existence. Neither drowsiness overtakes Him nor sleep. To Him belongs whatever is in the heavens and the earth. Who can intercede with Him except by His permission? He knows what is before them and what is behind them, and they encompass nothing of His knowledge except what He wills. His Kursi extends over the heavens and the earth, and preserving them does not tire Him. He is the Most High, the Most Great.
Source: البقرة: ٢٥٥ — أخرجه الحاكم ١/٥٦٢، وصححه الألباني في صحيح الترغيب ١/٢٧٣
قُلْ هُوَ اللَّهُ أَحَدٌ اللَّهُ الصَّمَدُ لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُن لَّهُ كُفُوًا أَحَدٌ
Say: He is Allah, the One. Allah, the Eternal Refuge. He neither begets nor is born, nor is there any equivalent to Him.
Source: أبو داود ٤/٣٢٢، الترمذي ٥/٥٦٧ — صحيح الترمذي ٣/١٨٢
قُلْ أَعُوذُ بِرَبِّ الْفَلَقِ مِن شَرِّ مَا خَلَقَ وَمِن شَرِّ غَاسِقٍ إِذَا وَقَبَ وَمِن شَرِّ النَّفَّاثَاتِ فِي الْعُقَدِ وَمِن شَرِّ حَاسِدٍ إِذَا حَسَدَ
Say: I seek refuge in the Lord of daybreak, from the evil of what He created, from the evil of darkness when it settles, from the evil of those who blow on knots, and from the evil of an envier when he envies.
Source: أبو داود ٤/٣٢٢، الترمذي ٥/٥٦٧ — صحيح الترمذي ٣/١٨٢
قُلْ أَعُوذُ بِرَبِّ النَّاسِ مَلِكِ النَّاسِ إِلَٰهِ النَّاسِ مِن شَرِّ الْوَسْوَاسِ الْخَنَّاسِ الَّذِي يُوَسْوِسُ فِي صُدُورِ النَّاسِ مِنَ الْجِنَّةِ وَالنَّاسِ
Say: I seek refuge in the Lord of mankind, the King of mankind, the God of mankind, from the evil of the retreating whisperer, who whispers into the hearts of mankind, from among jinn and mankind.
Source: أبو داود ٤/٣٢٢، الترمذي ٥/٥٦٧ — صحيح الترمذي ٣/١٨٢
It is known as the master of seeking forgiveness: the fullest wording for asking Allah's pardon, said once in the morning and once in the evening.
اللَّهُمَّ أَنْتَ رَبِّي لَا إِلَهَ إِلَّا أَنْتَ، خَلَقْتَنِي وَأَنَا عَبْدُكَ، وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ، أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ، أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَيَّ، وَأَبُوءُ بِذَنْبِي فَاغْفِرْ لِي فَإِنَّهُ لَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ
O Allah, You are my Lord; there is no god but You. You created me and I am Your servant, and I keep Your covenant and promise as best I can. I seek refuge in You from the evil I have done. I acknowledge Your favour upon me and I acknowledge my sin, so forgive me, for none forgives sins except You.
Source: البخاري ٧/١٥٠ (٦٣٠٦)
The two sets mirror each other: what you say in the morning — we have entered the morning — you say in the evening as we have entered the evening, with the same dhikr around it.
The app lists each chapter dhikr by dhikr with a counter for every line, so you can run them in order.
أَصْبَحْنَا وَأَصْبَحَ الْمُلْكُ لِلَّهِ، وَالْحَمْدُ لِلَّهِ، لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
We have entered the morning and the dominion belongs to Allah. Praise be to Allah. There is no god but Allah alone, without partner; His is the dominion and His is the praise, and He is over all things capable.
Source: مسلم ٤/٢٠٨٨ (٢٧٢٣)
أَمْسَيْنَا وَأَمْسَى الْمُلْكُ لِلَّهِ، وَالْحَمْدُ لِلَّهِ، لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
We have entered the evening and the dominion belongs to Allah. Praise be to Allah. There is no god but Allah alone, without partner; His is the dominion and His is the praise, and He is over all things capable.
Source: مسلم ٤/٢٠٨٨ (٢٧٢٣)
Every dhikr in the app is printed with the same takhrij that appears in Hisn al-Muslim — the book and the number of the report — so you can go back to any line you are unsure about.