Letrozole is one of the hormone therapy drugs..... it’s a non steroidal aromatase inhibitor...which means it works by lowering oestrogen production in the body, whereas chemo is a treatment to kill cancer cells. If your Mum has been prescribed Letrozole I would imagine her cancer is receptive to oestrogen ( ER+) . Most people take letrozole for 5 to 10 years as it helps to reduce the risk of a recurrence of the cancer.
Your Mum May or May not also have chemo.... maybe she could talk it through with her breast care nurse or oncologist. There’s a lot to take in, in the early days.