Nursery/preschool is not compulsory. Reception is the first year of formal schooling, you start the September you are 4 in 1st Sept (assuming England or Wales, different in Scotland).
The curriculum carries on from Nursery/preschool, but IME, much, much more emphasis on reading.My dc could recognise letters, their names and some basic words when they left nursery, and some phonics, but I’d never say that they were able to read. This seems to be the main difference with Reception.
Also, bear in mind that the term “Nursery” is also used to mean “private day care”. These places follow EYFS with children te right age, and often have children who come in just for this, for certain hours, but also have younger children and babies, and provide care typically 7.30 am -6 pm or so.
“Nursery School” or “Preschool” is usually associated with an Infant/Primary/First School, and runs for shorter hours during the school day and term time.
But reading is the big thing. Numbers, counting, simple arithmetic too.