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

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.