Official synopsis:What is the main plot for "Guardians of the Galaxy: Vol. 2" about?
Set to the backdrop of ‘Awesome Mixtape #2,’ Marvel’s Guardians of the Galaxy Vol. 2 continues the team’s adventures as they traverse the outer reaches of the cosmos. The Guardians must fight to keep their newfound family together as they unravel the mysteries of Peter Quill’s true parentage. Old foes become new allies and fan-favorite characters from the classic comics will come to our heroes’ aid as the Marvel cinematic universe continues to expand.
More unofficially (according to James Gunn and Kevin Feige:
Basically, the Guardians get called to a planet called The Sovereign where they are asked to destroy a monster in exchange for Nebula. Something happens there that causes Ayesha to hire the Reavers to get the Guardians. They're being chased by the Reavers when they encounter Peter's father.
Obviously, that last part is vague. I anticipate the full-length trailer will clear things up when that happens.