Each phase of films is loosely planned out before they start working on them as opposed to just going a movie at a time and hoping everything fits together in the end.
Seems like an odd thing to include in the marketing lingo. Seems more of an internal administrative thing.
It kinda made sense at first when The Avengers was the culmination of the previous “phase one” movies, as in like the finale of that season. Phase two and three have been like that to an extent but not completely. At this point it’s biggest use is to just compartmentalize the movies chronologically.