Yesca literally translates to "kindling" or "tinder", the dry material used to start a fire. However, in this song, it's used as a slang term.
In the lyrics, "No te enamores, después que te de yesca" (Don't fall in love, after I give you yesca), it implies getting someone high or giving them drugs, specifically marijuana. It's a street-smart word that adds a layer of edgy realism to the song's narrative about casual relationships.
Chesca and Jon Z's "Deja De Hablar (Blah Blah Blah)" is a super fun and direct song about being tired of empty words in a relationship. Chesca kicks things off, saying this guy tells her he loves her and she's perfect, but he never actually does anything. She's over the "blah blah blah" and wants action! She sings, "No necesito un hombre que me de palabras / Lo que yo quiero es que me muestre que me amas" [I don't need a man who gives me words / What I want is for you to show me that you love me]. She's not interested in just being taken to bed; she wants real effort.
Jon Z comes in with his perspective, which is a bit more playful and maybe a little commitment-phobic. He tells Chesca, "No te enamores, después que te de yesca" [Don't fall in love after I give you kindling (a spark)]. He admits he might not be able to love her, saying, "Baby lo siento, no puedo, confórmate con lo que hay" [Baby, I'm sorry, I can't, settle for what there is]. But Chesca isn't having it! She keeps pushing for him to "Come and put your money where your mouth is," demanding he back up his talk with actions. It's a great back-and-forth about communication, expectations, and getting real about what you want from someone!