Tendres means "tender" or "soft." While it might seem like a common word, its repeated use in this song, specifically with "bisous" (kisses), elevates its meaning and creates a very specific, intimate atmosphere.
In "Bisous tendres," the repetition emphasizes a gentle, affectionate, and sensual type of kiss, making the word itself evocative and central to the song's theme of soft, intimate moments in various settings.
Therapie TAXI build the whole song out of one instruction, repeated until it turns into a club chant: "Fait des bisous tendres" [Give tender kisses]. Each repeat moves the kisses somewhere new, "sur ses lèvres" [on their lips], "dans son cou" [on their neck], then "dans le club" [in the club] and "sur le sol" [on the floor], under "lumière rouge" [red light] and "lumière blanche" [white light]. The tenderness ends up on a sweaty dance floor.
The middle section drops the kisses for the heat of the room: "Qu'est-ce qu'il fait chaud, je veux de l'eau" [It's so hot, I want water]. Then the words fall apart into "Bi, bi, bi bisous bisous", the hook stuttered down to its first syllable, the way a crowd shouts it back.