Can you see i am so sorry if you can't not see

So a square is a special kind of rectangle, it is one where all the sides have the same length. Thus every square is a rectangle because it is a quadrilateral with all four angles right angles. However not every rectangle is a square, to be a square its sides must have the same length

He means that every square is a rectangle, as it has 4 sides, all four angles are right angles, and the diagonals are congruent.

However, not every rectangle is a square, as the sides have to be of equal length for it to be a square, but many rectangles have different widths and lengths, whereas a square has one length that is the same for all 4 sides.