What’s the difference between an FPS and an EPS?

Both are Real Time Information (RTI) reports you send to HMRC, but they cover different things:

Full Payment Submission (FPS) – Reports what you paid each employee and the PAYE tax and National Insurance you deducted. Zoho Payroll prepares an FPS automatically from each approved pay run, and you submit it to HMRC from Pay Runs.
Employer Payment Summary (EPS) – Reports employer-level information that isn’t on the FPS, such as statutory payments you’re reclaiming, Employment Allowance, or that you paid no employees in a tax month. You manage EPS filings from Forms & Filing.

In short: send an FPS every time you pay employees, and an EPS only when you have employer-level amounts to report (or a nil-payment month).