A right angled triangle (also called a right triangle)
is a triangle with a right angle (90°) in it.
The little square in the corner tells us it is a right angled triangle
(I also put 90°, but you don't need to!)
The right angled triangle is one of the most useful shapes in all of mathematics!
(It is used in the Pythagoras Theorem and Sine, Cosine and Tangent for example).