grain basket, bin for grain
囤 centers on a container for storing grain: a basket or bin specifically for grain.
囤 combines 囗 (enclosure) with 屯, which likely contributes the sound. The character represents a container for grain.
The outer 囗 is a container, and inside 屯 looks like a pile of grain. Picture a round basket filled with grain.
For トン, imagine a ton of grain stored in the basket: ton -> トン, and the basket holds a ton.