It's essentially an expansion DLC for the first game. It's not a direct sequel in that it doesn't continue the primary story from the first game that was teased at the end and I assume will be the plot of Spider-Man 2. It's a spin-off. Miles Morales exists in the same city as the first game where we saw how he got his powers, although it's prettier and winter now. Peter Parker from the first game exists and makes appearances in the game. But it's centered around Miles and is his story, not Parker's. Aside from Rhino (who is in the game's intro) and a brief appearance from one other prominent standard Spidey villain, this one is entirely Miles Morales based. It also references the first game's story quite a bit, so it'd definitely be better to play that one first.
Note: I finished the story but haven't done all the side stuff yet so it's possible other standard Spidey villains appear.