What is brackish water?

Brackish water refers to a water source that is somewhat salty but not as salty as seawater. The exact amount of salinity will vary depending on environmental factors and can not be precisely defined.

Is this a combination of fresh water and salt water when river meets the ocean?

Yes, the most extensive brackish water is when the river meets the sea. Which are estuaries.

Those are called estuaries?

When rivers meet the sea, or fresh water meets salt water, those are called estuaries.