It's the "all knowing eye" of the organization. In a generic sense, it's a software platform that brings together all the various parts of an organization - manufacturing, sales, HR, marketing, supply chain, inventory, accounting , ect and stores all of that info in a data warehouse for "easy" retrieval.
The goal is to end the frankenstein network of standalone specialized apps that many organizations use for the various departments.
Every company has a different way/level of implemenation though. Some may use pieces of ERP, some may use the whole thing. When you get into products that are enterprise in nature with stand alone modules for the various ancillary departments you really start compromising some of the functionality of specialized products. So YMMV by company on how effective they are.
Anyway...great job on the intership! ERP consultants make A LOT of cash. And if you are working with a defense contractor then you can maybe get a security clearance which makes you even more valuable. You are on an excellent career path!