Sablés are a plain butter cookie that can be consumed as they are with coffee or sandwiched into pairs with the likes of lemon curd, chocolate hazelnut spread or ganache. They are completely ubiquitous in France, and every family has their own recipe. The word 'sable' means 'sand', so the name refers to their 'sandiness' (which doesn't sound promising, so think 'crumbliness' instead). They are very 'short' - in cooking parlance - which means a high ratio of fat to flour, giving you a very soft pastry that tears easily and once cooked, has a soft, crumbly, melt in the mouth texture, a bit like Scottish shortbread.
You may be interested to know that one also 'sable le champagne' (breaks open the champagne).