Borderlands: The Pre Sequel tells the story of what happened between the two previous Borderlands installments (thus both "pre" and "sequel" i guess :)), but this isn't a game you really play for the story. Don't get me wrong, it's filled to the brim with hilarious moments, but after every quick laugh you'll be focusing on two things: shooting and looting. And this does a pretty good at both of them - Shooting is simplistic but fun and there are about a gazillion different guns, shields etc in this game thanks to random generation and mutliple different stats. Mind you it all get's a bit grindy though.
The game is set in space this time and this means you'll spend about half your time in low gravity environments. This is a double edged sword, as it adds the immensely fun "gravity slam" mechanic (which i will really miss in the next Borderlands game if they don't include it), but also makes the game feel a bit more sad and lonely than Borderlands 2.
Fans of the previous two games should definitely pick this up to see how jack and the other vault hunters come to be who they are and - of course - for the mindless fun that is the Borderlands shoot & loot formula :). New players should also join in in said mindless fun, but i'd suggest playing through Borderlands 2 first - you'll enjoy The Pre Sequel a bit more this way.