If rain is started thirty minutes after concreting, you don't have to take much tension as the initial setting time of cement is 30 minutes . But you have to cover the concrete with tarpaulin to prevent the flow of above fresh concrete

Concreting can be continued in light rain like sprinkling of water / spraying of water and the area where concrete is done to be covered with tarpaulin sheet immediately.

If there is a lot of rain and especially enough so that rain is washing across the top of the concrete it will be a problem, but you can certainly pour in light rain.

Water on the surface will impact the finish, so if the concrete is going to be an exposed sidewalk or polished floor you should avoid pouring in the rain.