An Interior Angle is an angle inside a shape
Another example:
Triangles
The Interior Angles of a Triangle add up to 180°
Let’s try a triangle:
90° + 60° + 30° = 180°
It works for this triangle
Now tilt a line by 10°:
80° + 70° + 30° = 180°
It still works!
One angle went up by 10°,
and the other went down by 10°
Quadrilaterals (Squares, etc)
(A Quadrilateral has 4 straight sides)
Let’s try a square:
90° + 90° + 90° + 90° = 360°
A Square adds up to 360°
Now tilt a line by 10°: 
80° + 100° + 90° + 90° = 360°
It still adds up to 360°
The Interior Angles of a Quadrilateral add up to 360°
Because there are 2 triangles in a square …
The interior angles in a triangle add up to 180° …
… and for the square they add up to 360° …
… because the square can be made from two triangles!
Pentagon
A pentagon has 5 sides, and can be made from three triangles, so you know what …
… its interior angles add up to 3 × 180° = 540°
And when it is regular (all angles the same), then each angle is 540° / 5 = 108°
(Exercise: make sure each triangle here adds up to 180°, and check that the pentagon’s interior angles add up to 540°)
The Interior Angles of a Pentagon add up to 540°
The General Rule
Each time we add a side (triangle to quadrilateral, quadrilateral to pentagon, etc), we add another 180° to the total:
So the general rule is:
Sum of Interior Angles = (n-2) × 180°
Each Angle (of a Regular Polygon) = (n-2) × 180° / n
Perhaps an example will help:
Example: What about a Regular Decagon (10 sides) ?
Sum of Interior Angles = (n-2) × 180°
= (10-2)×180° = 8×180° = 1440°
And it is a Regular Decagon so:
Each interior angle = 1440°/10 = 144°
Note: Interior Angles are sometimes called “Internal Angles”