。
If it rains tomorrow, I'll stay home.
Literal
Tomorrow rain [if-なら] I [topic-は] house [at-に] exist.
雨なら uses the なら conditional on the bare noun 雨 ('rain'). The longer 雨降り ('rainfall, rainy weather') would be a more vivid alternative; here the compact 雨 alone is enough, since context makes 'if it's rain[y]' unambiguous. The explicit 私は adds slight emphasis.