Sola fide, Latin for “faith alone”, is the doctrine that salvation is by grace through faith alone. There's nothing else we have to do to be saved. We don't have to do any works to earn our salvation, and indeed there's nothing we could do to make us more worthy of salvation. Salvation is gift only received by God's grace. All we have to do is repent and have faith in Christ to receive it.
It is largely based on the doctrine of total depravity. Fallen humans are incapable on their own of choosing God. It's only because God gives us grace to choose Him that we can be saved. We can't do anything to earn that grace, and we can't do anything to make ourselves more worthy of it. God described Israel's “righteous deeds” apart from Him as being like used menstrual products. Isaiah 64:6 Our attempts at being good without God are not attractive to God at all. It's only by His grace that we can be saved and through His power that we can do good works thereafter.