Also referred to as offer.
It is the lowest price at which a seller agrees to sell a financial asset.
Ask is a price by which you can buy currency being the first in a pair (XXX) and sell corresponding amount of currency being the second one in a pair (YYY).
Expression to sell XXX/YYY, for example, corresponds to selling XXX (first currency) and buying YYY (second currency).
Purchasing is always done by price Ask.