In trading, price is the most recent price at which a particular security was bought or sold.The price of the security is mainly determined by the supply and also demand.

Traders, Investors and Dealers interact in the market to determine the supply and demand which in turn indicates the market price of the security. So the price of the security is constantly changing  as the supply and demand quantities change. 

Bid represents Buying and it is the highest price at which a buyer  trader is willing to pay for a security.
Offer ( or Ask ) represents Selling and it   is the highest price which a seller trader  is willing to accept for a security.

Trading  transaction of a security occurs when the Bid and Offers coincides. 
Bid-ask (or Bid-offer spread ) is the difference between the bid price for the security and the ask price.
Exchanges provide a platform for traders to submit Bids and Offers through Brokers ( called market makers).

Practice Market Price – Beginner