Ask is an offer price, i.e. how much a seller is willing to accept for an asset. When you want to buy an asset, you look at the Ask price because this is how much a seller is ready to sell it for.
On a CFD-based platform, such as CEX.IO Broker, the Ask is the price at which you open your BUY (Long) positions and close your SELL (Short) positions.
The Ask is always higher than the Bid, and the difference between them is called Spread. The narrower the spread the easier it is to make a profit because the BUY order is placed at the higher end of the spread and SELL order - at the lower.