Almost every free quote you have ever seen is level 1. It answers what you would pay or receive right now for a small order, and nothing else.
Its blind spot is depth. Two stocks can show identical level 1 quotes while one has 50,000 shares behind each side and the other has 200. The first absorbs your order; the second gaps.
Example: both stock A and stock B show 10.00 / 10.01, 500 x 500. You buy 5,000 of each. A fills at an average of 10.011 because depth is stacked behind the offer; B fills at 10.09 because the next offers were 10.04, 10.07 and 10.12. Same level 1, very different market-impact.
Related: level-2, level-3-data, depth-of-market, market-by-price