As Owen points out, modest has the same meaning as unassuming, but it also has some additional meanings of its own.
Modest can a polite way of saying 'small': an unassuming house is exactly that - unpretentious, not flashy. A modest house, modest income, modest intelligence, these are simply small.
Modest dress is clothing which covers socially sensitive parts of the body. Different cultures have different rules about what is acceptable: to visit a church in Italy I would make sure my shoulders and knees were covered (and all the obvious stuff..).