The interior angles are those on the inside of the triangle.

Try this Drag the orange dots on each vertex
A,B or C to reshape the triangle. Notice that the interior angles always add to 180°

The interior angles of a triangle always add up to 180°. Because of this, only one of the angles can be 90° or more.
In a right triangle, since one angle is always 90°, the other two must always add up to 90°

A triangle is simply a polygon that has 3 sides. See interior angles of a polygon
for the properties of the interior angles of a polygon with any number of sides.

Properties

The interior angles of a triangle always add up to 180°

Because the interior angles always add to 180°, every angle must be less than 180°

The interior angles only add to 180° when the triangle is planar, meaning it is lying on a flat
plane.
If the triangle is not planar, for example if it is lying on the
curved surface of a sphere, the angles do not add to 180°.