If you are reliant on someone or something, you need them in order to manage. You are not standing fully on your own. We almost always say reliant on, not reliant to or reliant with. A sick person may be reliant on a nurse; a village may be reliant on one well. It names a state, and people often use it with a little worry: we have become too reliant on our phones. It is a bit more formal than we need or we depend on. Though actually, reliant and reliable look like close cousins — and they are, both growing from rely — but they point different ways. Reliable is the one others can trust. Reliant is the one who needs something else. Mixing those two is the easy trap.