In addition, not in contradiction to the other response, one or the other is favored more strongly by region. For example, my Mexican classmates would use aquí while my Dominican professor used acá. She said it was because of where she was from, and allowed her students to use the words interchangeably.
basically they are the same, but to try and help explain the difference compare "ven aquí" (come to the exact spot at which i am standing) vs "ven acá" (come here, i.e. next to me). Tbh this is not exactly correct but it is the easiest way I can explain it