Swap upper/lowercase
Paste a text: every uppercase becomes lowercase and vice versa.
The real case: Caps Lock left on
You've written half an email and then you notice it came out as «hI jOHN, hERE IS THE qUOTE»: Caps Lock was on and every Shift key did the opposite of what you wanted. Retyping everything is the natural reaction, but it isn't needed: swapping the case again puts the text back exactly as it was, because it's the same operation applied twice. Paste it, press the button and copy the result back.
The other case: the typographic effect
Swapped case is also used on purpose, in usernames, video titles and signatures, because it catches the eye without shouting the way ALL CAPS does. Be careful where you put it, though: screen readers spell out letter by letter anything that looks like an acronym, so in a long post or an important message it reads badly. It's fine for one word, much less so for a paragraph.
What changes and what stays
Only letters change, accented ones included: «Café» becomes «cAFÉ». Numbers, punctuation, spaces, emoji and symbols stay where they are, so the length of the text doesn't change and nothing is lost. This works for any alphabet, not just ours.
There is one exception that always surprises people, and it's better to know it in advance: the German ß would become two letters in uppercase, «SS», and from «SS» there's no way back. That's why here the ß stays as it is: «Straße» becomes «sTRAßE», and swapping again gives back «Straße». The same goes for the few letters that don't have a single partner, such as the Turkish dotted capital I and typographic ligatures: they stay where they are, so two swaps always give back the text you started with. And the Greek sigma, which is written «ς» at the end of a word, comes back as «ς» in the right place.
Nearby tools
If what you need is all caps, all lowercase or a capital initial, the right place is Upper and lowercase. For a capital on every word in titles there is Capitalize every word, and to rebuild a text typed entirely in lowercase Capitalize sentences. The alternating case of memes, that one on purpose, is Alternating case.