Short answer: A limited-purpose FSA only reimburses dental and vision expenses (and sometimes preventive care). Because it doesn’t cover general medical costs, you can pair it with an HSA without losing HSA eligibility.
A general-purpose health FSA disqualifies you from contributing to an HSA, because it counts as disqualifying coverage. A limited-purpose FSA (LPFSA) solves that: it restricts reimbursements to dental and vision (and, in a post-deductible variation, other care once the HDHP deductible is met), so it doesn’t interfere with HSA eligibility. Employees commonly use an LPFSA to pay dental and vision costs with pre-tax FSA dollars while preserving their HSA balance for other expenses or retirement. Both accounts have their own annual limits.