mas=pero, but
más=more

Más is = add

the meaning is much or more

MAS means but (in spanish it is also said as PERO) and it also means MORE.

E.g:

a)
Juan sabe como hablar español, más no sabe como hablar inglés.
Juan knows how to speak Spanish but he doesn't know how to speak English.

b)
Ana compró más papas que manzanas.
Ana bought more potatos than apples.

mas=pero (but), aunque(though), not very used today
más = plus, more