The estimated number of times a keyword is searched on Google per month.
Search volume tells you how big an opportunity is. 'Insurance' has hundreds of thousands of searches per month; 'auto insurance Peoria' might have hundreds. Higher volume = more traffic potential, but usually also = more competition. Search volume numbers from tools (Ahrefs, SEMrush) are estimates; directional, not exact.
Without knowing search volume, you're guessing at opportunity size. Target keywords with enough volume to matter but not so much that you can't rank. The 500-5,000 monthly range is often the sweet spot for new sites.