Well it actually is about the meaning. "Más" and "mas" are different words.
Más can be comparative adverb or a noun.
As a comparative adverb:
Ve más rápido.
Es el más fuerte.
Fuimos más de diez personas.
¡Qué casa más bonita!
As a noun, is mathematical addition "+":
Dos más dos igual a cuatro.
Mas is an adversative conjunction. This means it is a linking word used to introduce a contrast statement:
Hace frío, mas ayer hacía más. (Hace frío, pero ayer hacía más).
As you see, the first "mas" works as a linker introducing the contrast sentences, whereas the second "más" is the comparative, as it is comparing how colder was than today.
As a rule of thumb, write "mas" without tilde when you can substitute it by "pero" or "sino". The rest of cases will be "más".
Edit: As Carrigus Carraig comments, "más" is more frequent, you would never use "mas" unless you want to write something poetic, even in formal writing.