Deidad translates to "deity" or "god/goddess". It's a word with a strong, almost mythical connotation that isn't commonly found in everyday Spanish songs.
In the lyrics, Joan Sebastian sings about being replaced by "un par de amigos, un amante, un vividor o una deidad." The inclusion of "deidad" among more mundane replacements adds a touch of dramatic irony and hyperbole, suggesting that his replacement might be someone so perfect or revered that they're almost god-like, or perhaps he's just being playfully dramatic about the situation. This makes it a memorable and intriguing word choice.
"Que Sea" ("Let It Be") is Joan Sebastian's meditation on accepting a relationship's outcome with dignity instead of resistance.
He tells his partner that whatever happens between them, he will make peace with it: whether she stays or lets someone else take his place, whether it takes months or years to move on, he can adjust to whatever comes and be content with whatever love gives him in return.
Rather than begging or fighting for control, the song's resignation is its strength. He predicts she may one day regret leaving, but insists he already has the strength ("la virtud") to adapt to whatever she decides. It's a graceful surrender to fate in love, not defeat.