Explain absolute, relative and mixed references.

A relative reference like A1 shifts when copied — one column right becomes B1. An absolute reference like $A$1 never changes.

Mixed references lock one part: $A1 locks the column and lets the row move; A$1 locks the row and lets the column move.

Mixed references are what let one formula fill an entire grid correctly — a multiplication table uses =$A2*B$1. F4 cycles through the four states while editing.