A moment is a particular point in time. Some moments you remember forever: like the moment you earned your driver's license or the moment you learned that you passed your organic chemistry class.
A moment is a short, but unspecified, amount of time. At Thanksgiving, you are supposed to take a moment to be grateful for friends and family. Or maybe you have been asked to observe a "moment of silence" for a national tragedy. Moment can also mean "importance." If you say something is "of little moment," then you are saying it doesn't matter.