The word triangle is a combination of Tri - meaning three, and angle. But what does angle mean?
When two lines come together we sometimes call it a corner or a point. In geometry to be exact, we have a special name for everything. The pointy part outside is is called a vertex, and the inside open mouth part is called an angle.
One of the rules of squares and rectangles is that all the angles must be 90o (ninety degrees) which is like an L shape. If a triangle has a right angle it is called a right-angle triangle. This is one way to group triangles.
If an angle is smaller than 90o it is called an acute angle.
If an angle is bigger than 90o it is called an obtuse angle.
In the diagram below I marked two of the angles. Try to estimate how big the other angles are, and say if they are obtuse, acute, or right.