Bid price
The bid price represents the maximum price that a buyer is willing to pay for the currency. Meaning that in your perspective as the trader, this would be the price you sell.
Bid = Sell = Short
Ask price
The ask price represents the minimum price that a seller is willing to take for that same currency. Meaning that in your perspective as the trader, this would be the price you buy.
Ask = Buy = Long
Example:
As seen in this image, the BID price for EURUSD is 1.10710, meaning that you can SELL (short) the EUR at the price of 1.10710 USD. On the other hand, the ASK price for EURUSD is 1.10728 meaning that you can BUY (long) EUR at the price of 1.10728 USD.