Star-Lord infiltrates a bio-formed structure while Mantis reveals his abduction by Ravagers coincides with his mother's disappearance. Mantis suggests his grandfather's actions stemmed from a desire to protect him:
Star-Lord: What are you talking about?
Mantis: What?
Star-Lord: I'm talking about the people in my life who died, and you're talking about this?
Star-Lord points above
What does "this" in "about this" refer to?
As noted by Daniel Roseman in a comment, "this" is what Mantis has introduced to the conversation, namely that he's never gone back to Earth to visit his family, whereas Peter was previously talking about the people in his life who have died on him (not to mention that they're in the process of trying to get through the shields).
Peter: Everyone around me dies. My mother, Yondu, Gamora.
....
Mantis: Peter.
Peter: What?
Mantis: You had family on Earth, and you never wanted to go back to see them?
Peter: No, you’re my sister. That’s all the family I want or need. Besides, it was really just my grandpa, okay? He was a pretty hard guy.
....
Mantis: Still. Don’t you think he…
Peter: What?
Mantis: You were abducted by Ravagers the same day he lost his daughter.
Peter: Oh, yeah, he was upset. My mom died, he screamed in my face, he pushed me out of the room!
Mantis: He was probably just trying to protect you.
Peter: What are you talking about?
Mantis: What?
Peter: I’m talking about the people in my life who died, and you’re talking about this?
The raised finger is not meant to actually point to anything, but rather as a form of enumeration, indicating the topic that Mantis is raising. More generally, the scene is in part about how Peter likes to avoid painful aspects of his past. He really doesn't want to think about family left behind on Earth, particularly as it is associated with the death of his mother.
