Thin space

If it be only a Single Letter or two that drops, he thruſts the end of his Bodkin between every Letter of that Word, till he comes to a Space: and then perhaps by forcing thoſe Letters closer, he may have room to put in another Space or a Thin Space; which if he cannot do, and he finds the Space ſtand Looſe in the Form; he with the Point of his Bodkin picks the Space up and bows it a little; which bowing makes the Letters on each ſide of the Space keep their parallel diſtance; for by its Spring it thruſts the Letters that were cloſed with the end of the Bodkin to their adjunct Letters, that needed no cloſing.

Source: wiktionary

All you may see on the keyboard is a space bar, but typographers use several invisible characters: the word space, fixed spaces of various sizes (em space, en space, thin space, figure space, etc) and a hard space or no-break space.

Source: wiktionary