Decorators are nothing but annotations just like the one you use in
your apex class like @auraenabled which adds functionality to the
regular apex method to be able to be called from a lightning
component, but decorators here are annotations in the context of
There are 3 decorators specific to lightning web components:
To track a private property’s value and rerender a component when it changes, decorate the property with @track. Tracked properties are also called private reactive properties.
There is lot more documentation you can read about decorators and how to use them in the developer guide available in the component library.