In Star Citizen, concept sales are the sales of any items which is still under design, therefore, it is a non-usable item until the final inclusion in the game.
The prices of the concept sales are cheaper in comparison with the final prices those items will have once included in-game. Besides this, they normally have advantages such as exclusive improvements, lifetime insurances, or cosmetics add-ons.
Concept sales are limited in time, which means if you are interested in a given item, there's not gonna be a better chance to buy it.
The duration of a concept sale is usually around few weeks.