What is a testimony of faith?

A testimony of faith is when you tell people how you had a certain experience when they believed God or something helped them in a certain trial.

One of the most well known "testimony of faith" is the Shahada is Islam, where a believer declares "There is no god but Allah & Mohammed is his Prophet". In Christianity, a similar thing would be either the "Sinner's Prayer" or the moment where one first publicly renounces their sin and old life and declares their belief in God.