It's a common expression used to describe someone who is acting irrationally, foolishly, or in a way that seems out of touch with reality. It can be used in a playful or serious way, depending on the context and tone of voice.
Here are some additional points:
* "Loco" means "crazy" or "insane" in Spanish.
* "Tu" means "you" in Spanish.
* "Es" is the verb "to be" in the third-person singular form.
It's important to note that "tu es loco" can be considered offensive or insulting depending on the situation. Use it with caution and consider the cultural context before using this phrase.