It drops the evaluation from 6.4 (which is winning) to 2.4 (which is also winning).
In other words it's not a blunder you should worry about... in fact I wouldn't call it a blunder at all. The analysis tool is programmed to call anything that drops the evaluation by ____ amount a blunder... so that's what it does. In some cases it's not very instructive... like this case.
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Anyway, it tells you what move it liked instead of Bf4. It wants you to capture the knight. Probably because after black recaptures if you could put a queen on h5 that's already checkmate... in other words you capture the knight to weaken that diagonal, the kingside light squares in general, and specifically because you'll have tactics associated with Qh5 lurking.
* I kept looking but couldn’t find a vulnerability outside of the e4 pawn. It’s very early in the match and all pieces seem to be defended; so it’s hard to see Bf4 leading to anything critical - my eyes aren’t trained tho so..
Also, is there a resource you could recommend for understanding a question like this - blunders an whatnot.
Decode Chess is great but without a subscription you’re allowed one per day.