The quick answer: it’s a Jar opened from the Planning screen used for planning your budget in typical categories, or just to keep some money out of temptation’s way (so a Jar that’s not committed to a merchant featured on the app)
The details: When you create a Storage Jar you can set a goal for the amount you want to put aside, name the Jar and change its colour. Storage Jars don't offer the rewards (AGR) available with Hyper Jars.
Funds in a Storage Jar are e-money that is issued to you by our partner PFS (Prepaid Financial Services Ltd), which is an authorised Electronic Money Institution regulated by the Financial Conduct Authority in the UK. The e-money in Storage Jars can be moved in and out of them at any time, and you can set your HyperJar card to spend directly from a Storage Jar.