From the description:

Anyway, here’s the story of a support who’s secretly a DPS, and where the entire conflict stems from the fact that he never told his old party he was a DPS.

What does DPS stand for and what does it mean? Thank you very much!

DPS stands for damage per second.
So the player would mainly focus on dealing large amounts of damage to the enemy while their team could focus on other roles such as supporting and defending.

In MMO games you usually have three different roles : healer, tank and DPS. As said in the previous post DPS stands for damage per second but it's also the name given to the people who are dedicated to doing damage.

Tank is a guy with a lot of health and whose purpose is to take all the aggro (monster attention) from teammates so he's the only one getting it. Healer is the guy who heals people. Usually you'd have one tank, 1-2 healers and the rest of the people are DPS.