What is a promise?
You can say in simple words that promise is an accepted proposal.
When does the proposal become a promise?
Proposal becomes a promise when the proposal is accepted by the promise. If the proposal is accepted a contract is made, but if the proposal is not made then the there is no contract.
Mr. A proposes to Mr. B that he will teach him Economics and charge Rs 5000. There will be a contract if the Mr. B accepts his proposal. If Mr. B refuses then Mr. A cannot ask for the Rs 5000 as the proposal was never accepted and it did not become a promise.