The curly braces around a formula indicate that it is an array formula.
You do not type the braces yourself - instead, when you type in the
formula you finish (or commit) it by pressing CTRL+SHIFT+ENTER (CSE)
rather than just <enter. This automatically wraps { and } around the
formula if you do it correctly. If you click into the formula bar, as
if to edit the formula, the curly braces disappear and the formula does
not work, so you would have to do CSE again.

Hope this helps.

Pete