Pagination is a way for apps to divide big data sets into smaller subsets, or pages. Many apps use pagination to limit the amount of data they need to send at one time. This is done to prevent large data sets from using too many resources.
If you have 1 million invoices in your accounting app, a request for all invoices would require a lot of data. Instead, your accounting app can use pagination to limit the request to a smaller subset, such as the 100 most recent invoices.
Each app that uses pagination has different rules for how much and what data to send. Some methods include sending X number of data sorted by:
Custom parameters, such as SOQL queries for Salesforce.