Florece comes from the verb florecer, meaning 'to bloom' or 'to flourish'. It's a beautiful and evocative word that paints a vivid picture of growth and beauty.
In the song, the lyric "La alegría me florece" (Joy blooms in me) uses this word metaphorically to express how happiness blossoms and grows within the singer when his loved one is near. It's a poetic way to describe a profound sense of joy and renewal.
This is a plain, happy love song about the person who made everything easier.
The singer keeps returning to one idea. On his own none of it worked, and with her all of it does. Days he used to simply get through now fill up with sun, the road home is something he looks forward to, and he wants the whole world back again, as long as she is next to him.
What he actually asks for is small on purpose. Not grand promises, but Sundays, hugs, silly plans, kisses, and waking up beside her every day. The bridge says it outright: he does not want her for a little while, or for one summer and no further, he wants her in the simple things.
The title carries the whole argument. He does not know what it is about the sky, or about the world, but with her, yes.