Trigonometry
Trigonometry is math that uses angles and sides of triangles to measure things you cannot reach.
Think of it this way
You stand at one corner of your kitchen and want to know the room's width. You cannot walk across to measure it. Instead, you measure the angle from your corner to the far wall. With that angle and one side length, you can figure out the width.
Explaining trigonometry by grade level
Hold your arm out and point at a tall tree. Tilt your arm up to the top. That tilt is an angle. You can use that angle to figure out how tall the tree is.