Question

What is the difference between reading frame and frameshift mutation?

Answer 1

In a reading frame, one base is misplaced for another.

But in a frameshift mutation, a base is added or removed.
Remember, codons are groups of three, and they code for amino acids. In frameshift mutations, another base is added and that completely shifts the frame. Hence, the amino acids are messed up, and so is the protein made from it.