I know that:

bromophenol blue – used as a marker since most proteins and nucleic acid are colorless.

glycerol - It is a preservative and a weighing agent.

 

but what's the purpose of SDS and tris base in SDS agarose gel electrophoresis

It depends on what kind of electrophoresis you are doing. For proteins generally polyacrylamide gels are used instead of agarose. The reason why SDS is used is easily answered. In order for gel electrophoresis to work all the particles should have roughly similar charges (at least no inveresd polarity). Think about proteins, is that the case? For some other specialized applications SDS can have other uses including cell lysis in-gel.