The characters that leave no visible mark
A text string is not just the shapes you see on screen. It is a sequence of characters, and some of those characters tell software how to arrange or interpret neighboring text rather than appearing as a mark themselves. A zero-width space, for example, can indicate a place where a line may break without displaying a space-shaped gap. A joiner can affect whether adjacent symbols are rendered together, as in some emoji sequences.
An ordinary space is invisible in the sense that it has no ink-like shape, but it still occupies a position in the text. It separates words and can affect line wrapping, searching, and other operations. A formatting character may have a different job: it can influence layout while taking up no visible width.
Why the distinction matters
Software works with the underlying character sequence, not only the picture on the screen. Two passages that look identical may therefore behave differently when copied, searched, or edited if one contains an extra invisible character. A word with a hidden break opportunity, for instance, may wrap at a different point than the same-looking word without it.
This is why a blank-looking spot is not always empty. What appears to be simple spacing can encode a deliberate instruction to the text-rendering system—and that instruction can travel with the text even when the screen gives no obvious sign that it is there.
