Sea To Summit says:

Our collapsible X-Seal & Go Sets each include two different sized nesting containers ideal for snacks on-the-go, taking lunch to the office and for leftovers on the trail.

Made from food-grade, flexible silicone and a threaded airtight lid, they're simple to clean, easy to use and collapse to a fraction of their in-use size. For added compact storage, the smaller containers friction fit into the lid of the larger container.