Más and menos are adverbs that mean more and less. Note that Más is accentuated.
For example: Yo como más que tú. I eat more than you.
Yo tengo menos dinero que tú. I have less money than you.
But Mas can also be a conjunction, then it is written without accent, and it means “but”. It is equivalent to the word “pero”, which is much more used in colloquial Spanish.
Podría hacerlo, mas no lo haré. I could do it, but I won´t. Podría hacerlo, pero no lo haré.