Those red parts look like the expander rings
used in PEX radiant floor installations. You use a special tool to install them, and in general they must be cut off and- if things are going your way- you can heat the tubing underneath and twist out the (in your case) black fitting.
If things aren't going your way, you have to cut the fitting out of the tubing (that is, not remove the expander rings).
At least, that's what they look like to me.
There are several ways of securing fittings in PEX and similar plastic tubing. The Shark-Bite line is popular.