In short
Read a number plate at 20 metres (UK) before starting the practical driving test.
The eyesight test is the first part of the UK practical driving test. The examiner asks you to read a vehicle number plate from 20 metres (or 20.5 metres for old-style plates).
If you wear glasses or contact lenses for driving, wear them. Failing the eyesight test ends the test immediately — the examiner won't proceed to the driving section.