It literally means "to give oneself one's own taco." A person who se da su taco is someone who thinks is special and deserves special treatment. It is typically applied to a good looking person surrounded by not that good looking people.
Lic. Fernández: Oye, ¿por qué tu secretaria no contesta el telefono?
Lic. Hernández: Se da su taco porque fue la Flor mas Bella del Ejido alla en su tierra hace unos años. Cuando necesites algo, mejor háblame a mi celular.
Fernandez: Hey, why is your administrative asistant not picking up the phone?
Hernandez: She thinks she looks to good to be doing that, she won the beauty pageant back in her hometown a few years ago. If you want to contact me, you'd better call my cell phone.