Invitation to treat is when someone makes it clear they have something they are prepared to sell (most often the example is by putting something on display in a shop). They are inviting offers to purchase. The offer is when someone comes along (usually with money) and expresses an interest/intention to purchase. The common misapprehension is that an invitation to treat is binsing (as per when a shop puts something on display with the wrong price on it). It is not and the seller can refuse to accept the offer even if the offer is made at the price asked for.