As per Stripe, 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.