Sola fide means "faith only" in Latin. It is used by Protestants, who believe that people must trust Jesus to bring them back into relationship with God. Sin creates a gap between people and God, but Jesus, who was perfect, died by taking the punishment for sin for the people and so bridged the gap. That is called salvation because Jesus "saves" people from being without God. The idea is that trust in Jesus is the only way to be saved and to get to heaven after people die.
That belief was a principle of the Protestant Reformation, which was started by Martin Luther, and is a principle of Protestants today (see five solas). It contrasts with Catholics and the Eastern Orthodox, who believe that Jesus died to bridge the gap between God and people, but they must also behave to be saved.
Protestants agree that people need to do what is right, but they believe that salvation depends on faith alone, not on acts.