In stock trading bids can exist at several price levels below the market price, which is where the current price of the stock is. When people say ‘bid,’ they usually mean the highest bid price.
Do I get filled when I bid at the market price?
Not necessarily, the market price is the price of the last transaction that occurred. When you place a bid at the market price, you need to wait until someone is willing to sell to you at that price.
What happens if I bid at the ask?
The ask is the price someone is willing to sell his shares. If you bid at the ask, you will get immediately filled (if someone is not faster than you) as long as there are enough shares to sell at that price. If there are not enough shares, your order will get partially filled, and you need to wait until someone is willing to sell you more at that price.