A preset is the speed setting you run an encoder at - a named bundle of settings that trades encoding speed for compression efficiency. In x264 and x265 the presets run from ultrafast through medium (the default) to placebo; SVT-AV1 uses a numeric dial from about 0 (slowest, most efficient) to 13 (fastest), but the idea is identical. The preset does not change the quality target you ask for; it changes how hard the encoder works to hit it, since a slower preset is allowed to test more ways to encode each block - more reference frames, a wider motion search, fuller rate-distortion analysis - and keep the best. Each extra tool finds a few more bits to save and costs time. The catch is fairness: comparing a slow preset of one encoder against a fast preset of another, then blaming the gap on the codec, is the classic unfair benchmark. Comparisons are only valid at matched effort, which is why the exact preset must be recorded.
