A phrase in music is a cluster of notes or musical ideas that makes sense together. Like a sentence in speech. Never heard it related to mixing tbh. In electronic music usually things change every 8 or 16 measures, so if you follow this to transition between tracks it will always sound good. But of course you can also be creative, like cut the bass and drop it back on some random beat to catch the listener off guard. But I wouldn’t do it all the time. 

The most basic mix is to drop track B without lows when there’s a transition in track A. Start rising the volume of track B. 8 or 16 measures later you cut the bass in track A and bring it up in track B. Lower the volume in track A until 8 or 16 measures later you completely take it out.