Lacrou is slang that corresponds to a praise for who did very well in something that left others speechless or unresponsive.
Whoever sealed it did not leave room for them to speak badly, meaning that that person "shut up" the others, as in the phrase "sealed up the enemies".
That is, if it sealed it is because it used good arguments to ensure that it is not discredited.
Sealed slang emerged as a catchphrase of the youtuber Romagaga in 2013, and was popularized among the gay community as an internet meme, until it reached the popular vocabulary.
Learn more about the meaning of Meme.
Seal in slang finds its synonym in the word arrasou, but meaning a little less intensity in "seal", that is, in argument or action.
Sealer is the one who sealed it. The term is used only in females. The male correspondent, sealer, is not used as slang.
In the formal Portuguese language, lacrou is the perfect past tense of the verb lacrar in the third person singular. It means to close something with a seal, that is, to hermetically seal, to insulate.
See the meaning of Seal.