Refers to rain that is not heavy; often contrasted with 大降り (heavy rain).
See also: 大降り
It's only drizzling today, so I think I'll be fine without an umbrella.
小降りの雨が一日中続いている。
A light rain has been falling all day.
Compound of 小 (small, light) and 降り (rainfall, from 降る).