A Season in STO doesn't suscribe a bunch of story episodes necessarily.
A season update is more like big code/system expansion that can include new content.
S7 is: the new Tau Dewa Sector Block including many patrols, the New Romulan ground adventure zone and the Reputation system (Omega Force and Romulan) and the Fleet Embassy on New Romulus
S6 was: mostly the Fleet Starbase System
S5 was: The Free to Play conversion
S4 was: For the most part the Ground combat revamp and many things and centent that never happened.
Cant really remember what was part of the earlier seasons, but you get the idea.
To play the new Romulan Content in Tau Dewa you have to reach level 50.