To rely on someone or something means to depend on them or trust them to do what you need. When you rely on your alarm clock to wake you up for school, you're counting on it to ring at the right time. When your teacher relies on you to feed the class hamster, she trusts that you'll remember to do it.

Relying on someone is about trust and dependence working together. You might rely on your best friend to keep a secret, or rely on your older brother to help with homework. A basketball team relies on its point guard to make smart decisions with the ball. Scientists rely on careful measurements and experiments to discover how the world works.

The word means you're building your plans around an expectation that something will happen. If you rely on catching the 8:15 bus, you're depending on it being there. If the bus is reliable, it shows up consistently and you can count on it. Someone who is reliable does what they say they'll do, which is why people feel comfortable relying on them. When people prove reliable over time, others naturally begin to rely on them more.