It refers to price rate a bidder (or a buyer) is willing to pay for a particular product.

Bid refers to an amount a buyer or a bidder is willing to offer in return to a product. In online advertising world, bidding is done on things like Google Adwords and so on. Bidding as such is a sophisticated process and requires strategies in place to make it work for you. However, right bidding and at right time can ensure you rich dividends within a short span of time. If you are bidding for Adwords, for example, then your first strategy would be to predict what your income goals are and how you are going to create a list of efficient keywords and so on.