In poker, what does HJ ad CO (next to a player's name) mean?

They're seating positions at a poker table. HJ stands for Hijack and CO stands for Cutoff.

The cutoff is directly to the right of the button and directly left of the hijack:

http://www.grinderschool.com/articles/texas-holdem-position

HJ is short for hijack or hijack seat. CO is short for the cutoff position.

In poker the person with the button in front of them is considered the "dealer". This person will always be last to act post flop when they are in a hand. This gives them a position advantage. The player to the right of the button is the cutoff and the player to the right of the cutoff is in the hijack seat. The position got it's name because players in the hijack seat can steal the late position advantage of the button seat by making an opening raise.

https://learn.pokernews.com/pokerterms/hijack.htm