Pesadão is an augmentative form of pesado (heavy). It means something is extremely heavy, powerful, or intense. It's a colloquial and impactful word that adds a lot of punch.
In the context of the song, IZA uses "o bonde é pesadão" to describe her crew or
IZA opens by naming where she started and where she stands now: "Mas veio lá do poço" ["But came from the bottom"], set against "Noix tamo no topo" ["We're on top"]. The title phrase, "linha de frente" (front line), runs through the whole song as her claim to the lead position — she made it there on her own terms and isn't giving it up: "Valorizei minha voz então fique com o troco" ["I valued my voice so keep the change"].
The verses stack up the proof: designer brands, luxury cars, and a line that turns her success into a literal record — "O poder de compra da minha voz entrou para o Guinness" ["The purchasing power of my voice entered the Guinness"]. But she's careful to say the rise didn't erase where she's from: "Saímos do gueto mas o gueto nunca vai sair de" ["We left the ghetto but the ghetto will never leave from (us)"]. The chorus repeats "O bonde vai chegar e ninguém vai passar" ["The crew will arrive and nobody will pass"] like a chant, and by the bridge the warning turns playful, stretching "pesadão" ["heavy/hard-hitting"] out into "pesadão-dão-dão" for emphasis — the sound of a crew that isn't slowing down for anyone.