The Stash is where you can store Tools and Resources. After you initially repair the stash at the beginning of the game, you will find the Axe, the Trowel, and a stack of 4 tomatoes inside of it.
When you Use the stash, you will be presented with a window. This window will show slots for your Tools and slots for your Resources. Your stash can always store up to 4 Tools at a time. At the beginning of the game, it can store 2 stacks of Resources.
Upgrading the Stash[]
You can upgrade the stash. Each upgrade changes the appearance of your stash and the number of Resource storage slots, up to the maximum number of six slots.