Supply is the willingness and ability of sellers or suppliers to make available different possible quantities of a good at all relevant prices.
Supply is what we have to offer. All of us have our time and skills to offer to our employers. For some of us, the number of hours of work may change from day to day or from week to week. Then, most often, if additional hours are required to be worked, we can expect a higher price, i.e., overtime pay.