It tells me that your mare is heterozygous (homozygous would through tobiano everytime). The coat pattern is determined by what is given by each parent.
Tobiano can be heterozygous which means the horse carries only one Tobi gene (and will pass it 50% of the time), and homozygous which means a horse that carries two tobi genes and will pass one 100% of the time.
Overo is alittle more complicated, partly because there are more then one type of overo, and because some forms (maybe all, I don't know) can hide, and a horse can look solid.
If you breed hetero tobi to overo, you may get a tobi, overo, or a combo of both, or you may get a solid foal.
Hope that helps.