What kind of wire connectors are these?

TP is the designator for Test Point.

The plastic part just moves the loop part off the board, but you can see some without the plastic part. Nothing more than a small looped section of conductive wire. They can function as jumpers at the same time.

They are used for test clips to grab onto.

Some other options are 2 dimensional holes:

They are test points, as shown here.

They are suitable for mini-grabber connections to lab equipment or probing

As others say, these are test points. The coloured plastic rings can be used to indicate the type of signal on that test point. On the board in your picture, it appears that black testpoints are Ground. Red is for power - I see one labelled "+3.3V".