ERP is generally referred to a class of business softwares that addresses a business firm in totality, rather than one or two functions.
For example, there are accounting softwares, inventory softwares, etc. that addresses a specific function of a business. But an ERP class software adrresses all the functions of the business and provides an integrated view of the business to the management.
SAP is a German company and is the world leader in ERP softwares across various industries.
ERP attempts to integrate all departments and functions across a company onto a single computer system that can serve all those different departments' particular needs.
That is a tall order, building a single software program that serves the needs of people in finance as well as it does the people in human resources and in the warehouse. Each of those departments typically has its own computer system optimized for the particular ways that the department does its work. But ERP combines them all together into a single, integrated software program that runs off a single database so that the various departments can more easily share information and communicate with each other