What is a brace in soccer?
When one player scores two goals in a single match. Not as rare as a hat trick (three), but still a huge day. The term comes from an old hunting word for a pair.
Explain further
The one-liner gives you the headline, so here is the rest. A brace is simply two goals by the same player in one game, and it counts no matter how those goals arrive or how far apart they fall. Two penalties, two headers, one in each half, two in the final five minutes, an own goal does not count for the scorer but everything off his own boot does. If the same name lands on the scoresheet twice, that is a brace. There is no special rule attached to it; it is just the word commentators and fans reach for instead of saying "scored twice."
Here is the part that trips up Americans new to the sport: the game has tidy names for these scoring milestones, and they climb in rarity. Two goals is a brace, three is a hat trick, and four or more has no single agreed-upon term, so people usually just say "four goals." The word itself is borrowed straight from British soccer culture.
That term comes from old hunting, where a "brace" meant a pair of birds bagged in a day. Soccer kept the flavor of that, two of something gathered up. So when you see "Mbappe with a brace," you now know he found the net twice and very likely had himself an excellent afternoon.