Be aware that Page Composer is being deprecated and step-by-step replaced by App Composer. How far along this is will depend on which product(s) you're using.
(Not that App Composer currently supports any field layout features!)
Where can I find good documentation on App Composer?
Page composer is used to modify user interface pages like Hide or Show field,Make the field required or not , Change field label and so on,
To Use Page composer you need to activate the sandbox and check if either the Edit Pages or the Edit <Page Name> Pages menu item is available in the "Settings and Actions" menu. If no, then that means the page can't be modified.
Refer the below documents to know more about Page composer,