Answering the Testimony in the Adhan
وَأَنَا أَشْهَدُ أَنْ لَا إِلَٰهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، وَأَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ، رَضِيتُ بِاللَّهِ رَبًّا، وَبِمُحَمَّدٍ رَسُولًا، وَبِالْإِسْلَامِ دِينًا
Wa ana ash-hadu an la ilaha illallahu wahdahu la sharika lah, wa anna Muhammadan 'abduhu wa rasuluh. Raditu billahi rabban, wa bi-Muhammadin rasulan, wa bil-islami dinan.
And I too bear witness that there is no god but Allah alone, with no partner, and that Muhammad is His servant and Messenger. I am content with Allah as my Lord, with Muhammad as my Messenger, and with Islam as my religion.
What it means
The muadhdhin calls out the testimony of faith and you answer as a witness yourself, adding that you are content with the arrangement. Five times a day you are given a chance to re-declare your allegiance out loud.
When and how
Say it quietly after the muadhdhin has finished the two testimonies. For the rest of the adhan, repeat his words exactly, except after Hayya alas-salah and Hayya alal-falah, where you say La hawla wa la quwwata illa billah.
Its virtue
The Prophet ﷺ said that whoever says this when he hears the muadhdhin give the testimony will have his sins forgiven, reported in Sahih Muslim 386.
Source
Muslim 386sahih