Tóxico directly translates to "toxic" in English. It's an increasingly common word in modern Spanish slang, especially in discussions about relationships and personal well-being.
In this song, the artist uses it to describe himself, saying "Me dijo 'tóxico'" (She called me 'toxic'). This self-awareness, or perhaps a reflection of his partner's perception, makes the word stand out as it hints at the complicated and potentially unhealthy dynamics of the relationship portrayed in the lyrics.
"Te Dijeron" is Eladio Carrion's confession that karma finally caught up with him. Your friends warned you he would let you down, and he did exactly that: chasing you, breaking promises, then walking away only to come back for more. Now the roles have flipped. The person he hurt has moved on and gone cold, and he is the one left anxious, checking his phone, hating how much he still wants her attention.
He owns the damage. He calls himself toxic, admits he brought this on himself, and knows he cannot undo what he already did. Under the bravado there is real regret, a reminder that the person who plays games eventually ends up on the losing side of one.