There are several actions in poker called declaration, in which a player formally expresses his intent to take some action (which he may perform at a later point). For example, one may verbally declare an action (fold, call, raise) while in turn, which obligates the player to complete that action. One may declare a number of cards to draw in a draw poker game (which is typically not binding), or one may declare some other choice specific to the variant being played.
But most commonly, the term refers to the declaration in the final phase of a high-low split game, in which players indicate whether their hands are to be evaluated as high hands, low hands, or both at showdown. This is only one option for high-low split games; the other is known as "cards speak", in which players simply reveal their hands at showdown and award the pot to the highest and lowest hands shown (possibly subject to qualifications). Cards speak is used commonly in casinos because it is the much simpler method. High-low with declaration is common in home games.