Cheaper is the comparative form of the adjective "cheap", meaning costing less money than something else.
Common usage examples:
1. "This phone is cheaper than that one."
2. "We found a cheaper alternative to the original product."
3. "It's much cheaper to buy in bulk."
Other meanings:
1. Of inferior quality (often used with "look" or "feel")
2. Requiring less effort (e.g. "a cheaper way to do it")