A schema (schemas or schemata in plural) are representations of things in the form of an outline or model. It is a way of structuring a representation.
The word schema has its origins in the Greek skhēma meaning ‘form, figure’.
Schemas are used in data modelling to represent data types, often representing real-world things.
You could define a simple schema for an event, having: a title, a start date, and a venue.
This would allow you to describe the event in a predictable way so that a machine or a program could read it later. This is particularly useful if you think you will end up with more than a handful of events.
You could then later query this structured data by saying something like give me all events with start date later than today. And you’d get a list of events after now.
See schema.org.