Think of it this way: Your child wants to know if you mean what you say -- and say what you mean. Are you reliable? Trustworthy? Do you stand by your words? Being a reliable parent -- in your child's mind -- is at least as important as the actual limit you are setting. Toddlers crave this sense of security as they start to venture out into their new, wild world.
That's why it's important to be consistent in parenting. You don't have to be inflexible -- you can re-set limits if there's a good reason -- and explain to your child WHY you've changed your mind. But staying consistent proves to your child that you're reliable, predictable, and SAFE. And nothing helps your child blossom more than that.