Reserved funds are used to prevent negative balances due to disputes and refunds. Most reserves Stripe places are temporary. There are a number of reasons that we might place a reserve on an account, but these are some of the most common:
Stripe requires additional insight into your normal charge behavior at onboarding;
Your account is experiencing high chargebacks or dispute rate;
Your business operates in an industry or with a business model that creates additional credit risk;
Your business collects funds far in advance of delivery of goods or services.
The Stripe Services Agreement has more detail on how we execute reserves.
For more information on how reserves are applied and removed, see the Reserves topic in the Support Site.