ERP stands for Enterprise Resource Planning
Wikipedia defines an ERP as "... the integrated management of core business processes, often in real-time and mediated by software and technology ..."
Most companies define an ERP as "... lots of money, lots of effort, lots of time, lots of headache ..." It doesn't have to be. Any big change is a little stressful, but with proper planning, an intuitive application and good employee training you can mitigate most of these.
Most businesses (at least the bigger ones) in the past had some sort of processes that they used to manage activities. All of this was documented on paper, stored in folders, moved along in memos, shared as bound books and so on. Paper was ubiquitous. Then technology arrived.
When technology started making an impact on businesses, most companies hired consultants to create solutions for small problems they faced. These problems included actions like printing invoices, tracking inventory, scheduling jobs, processing payroll, etc.
With time, there were as many independent solutions as there were problems - which is a problem in itself. The various software solutions worked really well but they didn't talk to each other, or use data from each other. Keeping various departments and activities of a business in sync was the next big problem.