Prejudicado means 'harmed' or 'disadvantaged'. While it can also mean 'prejudiced' in some contexts, in this song, it refers to someone who is suffering the negative consequences of a situation.
João Gomes uses this word to describe the person who is left behind after a breakup, singing, "É que o abandonado sai prejudicado" (Because the abandoned one ends up harmed). It highlights the emotional toll and unfairness felt by the one who is suffering.
João Gomes opens with a quick shoutout to the promoters and hosts before the song really starts, a common touch in Brazilian piseiro and forró shows.
The song itself is about the uneven pain of a breakup. The singer says his heart is tired and his sadness fills every corner of the room he's left in. He points out that ending things might have worked out fine for the other person, but not for him: whoever gets left behind is the one who really comes out worse off.
That person ends up drinking at bars, spending money they don't have, and sending messages that never get a reply. The chorus repeats the idea plainly: it's the one who lost someone who ends up hurting.