1. To make a substance flow from a container, especially into another container, by raising just one side of the container that the subtance is in:
I spilled the juice while I was pouring it.
2. to (cause to) flow quickly and in large amounts:
The bus was pouring out thick black exhaust fumes.
3. If you say about the weather that it is pouring, you mean that it is raining heavily:
You’d better take an umbrella – it’s pouring out there.
https://dictionary.cambridge.org/dictionary/english/pour
In Spain we have an expression for the third meaning:
Pour with rain = Llover a cántaros.