Single-tapping is a tapping style for osu! in which players click circles, slide sliders, and spin spinners using only one key on their keyboard (Z or X by default). This style is common amongst players of all skill levels and is the easiest to adapt to.
Despite the name, most single-tap players do not always tap with one key. Most prefer to single-tap most everything except for fast streams, stacks, and burst streams. In which case they will often opt to alternate and/or double tap.
Arguments in favour of single-tapping typically mention that it is easier to keep consistent because there is — for all intents and purposes — no second finger to keep track of. And typically this is the reason (conscious or not) many newer players opt for this method.
By the same token, many players argue that single-tapping is slower because there is a necessary delay that is needed before you can press down that same finger, while others argue that using only one finger can cause the user to get fatigued much more quickly.