Karlach is one of several different companions that you can recruit in Baldur's Gate 3 to be a member of your party and help in your fight against the Absolute, but like all of the companions she has her own personal quest that you can complete to get a better ending for everybody. Karlach's might take a little longer than other quests though, and that's where we come in.
Karlach has an overactive heart, made from materials found in the hells of Avernus. This means that she cannot touch anybody and will periodically light on fire, but there is a way to fix her. You need to find a special material that's dotted around the Forgotten Lands called Infernal Iron. You'll likely come across this material naturally in your travels, especially considering how much you need to actually help Karlach out.
Once you have the Infernal Iron though, that's where it becomes slightly more complicated. You see, if you've already completed the quests involving the Druid Encampment then the mechanic that you need has already moved, and you can't reach him until Act 2. That means you'll need to hold onto your Infernal Iron until you've found him and then you can give them to him. Once he has two pieces, he'll make it so that Karlach can actually touch people again, which can help advance her romance, so there's a reason that you need to do this if you're looking to romance the muscular red Barbarian.