You may have seen this amp or that guitar amp described as having a lot of “clean headroom” or heard that high-wattage guitar amps have more “headroom” than lower wattage amps. Headroom is a simple concept, but often misunderstood. How important headroom is to you depends on how you use your amp.
“Headroom” (an automotive term literally referring to how much room there is between the top of your head and the roof of the car) as it relates to amplifiers refers to how far you can turn up an amplifier before its clean tone begins to distort. Amps that are designed to distort quickly upon turning up the gain and/or volume knobs have low headroom. Amps that stay clean at higher gain/volume levels are “high headroom” amps. In another sense, amps with high wattage, even if they distort early on their gain knobs, have more headroom by virtue of the fact that you generally keep the volume at a tiny fraction of their full wattage, unless you’re playing a football stadium.
Whether high headroom is important or not depends entirely on how important getting a clean tone out of your amplifier is to you. Some players buy low-wattage amps specifically because they want to get distortion at lower volumes. High headroom doesn’t matter at all in this application. Others prefer high-wattage amps, but want to get high-gain tones. However, if you mostly play clean or edge-of-breakup sounds, you definitely will want an amp with a lot of headroom so you don’t need to worry about your amp distorting when you don’t want it to.