。
If it rains tomorrow, I won't go.
Literal
Tomorrow rain [subject-が] fall-if I [topic-は] go-not.
雨が降れば uses the ~ば conditional on 降る ('to fall/rain'). Where なら typically attaches to a noun (雨なら 'if it's rain'), ~ば attaches to the verb itself (降れば 'if it falls') — slightly more dynamic in feel, since the ~ば form treats the rain as an event rather than a state of affairs.