Rrain-té is a French slang term, a verlan inversion of "terrain" (territory or ground), often used to mean one's neighborhood or 'hood'. It's a word that immediately signals a certain urban, street-level context.
In the song, GIMS uses it to assert his presence and dominance, saying "J'arrive sur ton rrain-té" (I'm arriving on your territory). This phrase implies a confident, almost confrontational arrival, making it an engaging and memorable word that captures the song's energy.
GIMS's "Soleil" is a super confident track where he's basically saying he's unstoppable! He starts off talking about how he's been dropping hits for years, saying "J'envoie bangers depuis des années" [I've been sending bangers for years] and that he's going to "désarmer" [disarm] all the other DJs. He's wondering if he'll get the exclusive this year, showing he's always aiming for the top.
The chorus is all about the power of "Soleil sur ton corps, c'est fort" [Sun on your body, it's strong]. This imagery of the sun on a body is so powerful it "ouvre toutes les portes alors" [opens all the doors]. He's going to hold onto this feeling, or perhaps a person, "sans l'abîmer" [without damaging it]. He even talks about arriving "sur ton rrain-té" [on your turf] with full force, saying "J'arrive pleins phares, donc j'peux pas rater" [I arrive full headlights, so I can't miss]. It's a bold declaration of his presence and success, mixed with a little bit of playful swagger about what someone might be looking for in a partner: "un mec plus stable, un mec plus fiable" [a more stable guy, a more reliable guy] who brings in money.