mentaly: "Your mother", "your" form of netaly, "his/her mother" (Tiipay/non-'Iipay dialects). Here with nominative suffix -ch, marking mentaly as the subject
Waam: "Goes away, leaves". Here, with future suffix -h, waamh: literally, "Will go/leave, is going to go". Waamh usually translates to "is going (now)", so the sentence means, "Your mother is going (i.e. leaving, now)" or "Your mother is going to go/leave (imminently or in the future)". By contrast, "Mentalych waam" would likely translate to, "My mother left/went" or "My mother goes (habitually)."