Assuming a Premise is True

I have heard many times that arguments are valid if the conclusion logically follows the premises, assuming that the premises are true. This I understand. However, in an example used in the Intro to Logical Reasoning video, the video presenter says that an argument is invalid on the basis of the premise being false. However, I thought we are to assume that premises given to us must be true? So, my question is, must we assume premises are always true? Or is it possible that some premises being false could make the argument invalid?