Question

Can someone tell me what the small circles on the angles of the triangles mean and what they are called?

Answer 1

those are angle "tickmarks", usually you'd also have tickmarks on segments denoted by a small line or two small lines, meaning, those sides are equal in length, in this case, it means those angles are equal. 

so, in this case, ∡ G is equal to ∡ D, and ∡E equals ∡H and ∡F equals ∡I.

Answer 2

The semi-circles in the triangles represent the verteces or different angles on certain points of the triangle.

-Hope this helped! :D