If you've used a reward chart or jar can you explain it to me?

Does your toddler / preschooler understand it? Where do you keep it?

I'm thinking about doing a reward jar for bedtime. She's been awful about leaving her room lately and I'm at a loss at what to do. I'm thinking of getting a small jar and filling it with marbles (or something similar) for each night she stays in her room. Taking it away if she leaves then when she fills the jar she gets a reward.

I'm not sure she'll understand waiting for the reward though??