Let Him Run Wild Lyrics
Tears filled my eyes
And when I heard you talk with him
I couldn't stand his lies
And now before he tries it
I hope you realize it
He don't care
Let him run wild
He'll find out
Let him run wild
He don't care
He'll do the same to other girls
That he did to you
But then one day he'll run in to one
That's gonna hurt him too
Before he makes you over
I'm gonna take you over
He don't care
Let him run wild
He'll find out
Let him run wild
He don't care
All the dreams you shared with him
You might as well forget
I know you need a truer love
And that's what you'll get
And now that you don't need him
Well he can have his freedom
He don't care
Let him run wild
He'll find out
Let him run wild
He don't care
Let him run wild
He'll find out
Let him run wild
He don't care
Let him run wild
He'll find out
Let him run wild

The song has an immature, moralistic, and jealous mindset. PoprocksCk has the meaning correct. And it's a common, bitter trope that's sexist, maybe misogynistic.
He loves the girl from afar. He's waited for her. She is going out with someone else though. His jealousy is triggered by their physical relationship, and he can only perceive it as him manipulating her, taking advantage. He can't accept that she might legitimately like him, or make her own decisions about her body
The other guy might be a player, and in the mind of the narrator that's all he is. But he clearly has something the narrator doesn't have. Most likely it's confidence, because he doesn't just stand around and wait. It might even be that he appreciates the girl for who she is, and doesn't view her as an exalted princess on a pedestal.
So the narrator is fantasizing that the guy has hurt her and dumped her, and started going out with other girls - running wild. She will then obviously come to realize that the narrator was the one with true love. And eventually the guy will get his heart broken, revenge served cold.
But let's be forgiving of our poor narrator because this song came out in 1965, before the Sexual Revolution really got going. And it's easy to forgive when the song is truly a great one. Those rich Beach Boys harmonies, with Brian Wilson's beautiful high falsetto leading the way. This song is as thrilling and beautiful as the Beach Boys get.
[Edit: Tone down]

What a great song. One of the more underrated songs in the Boys' catalogue. I guess it's about a guy who really loves a girl and wants to be with her, but the problem is that the girl is drawn to another guy. Now, he doesn't tell the girl at first, but he knows that this guy is just the manipulative type who just wants to get with as many girls as possible and just uses his charm to lure them in, trick them into thinking he loves them, and then ditch them like yesterday's news. The narrator is confident that one day this guy will realize the errors of his ways, but that time is not now and this girl is not the one that's going to break his vicious cycle. We just have to let him run wild for now, and one day he will reach the same level of maturity that the narrator is at NOW. The guy apparently cheats on her or does something to hurt her without officially dumping her. The narrator then tells her that he knew this would happen all along, and that now that she finally has some way to see what the narrator means, she should leave the other guy and get with the narrator, since he's the one that truly loves her. And it'd be a perfect situation for everyone involved - the narrator and the girl find love, which is what they want, and the narrator can continue to run wild and continue his manipulative ways, which is what he wants... or is it?