A Pass is a single rendering pass, ie a single draw of the given material. Note that if you create a non-programmable pass, during compilation of the material the pass may be split into multiple passes if the graphics card cannot handle the number of texture units requested. For programmable passes, however, the number of passes you create will never be altered, so you have to make sure that you create an alternative fallback Technique for if a card does not have enough facilities for what you're asking for.
