An ERP joins up the core departments of a company so they work from the same information. When sales confirm an order, purchasing can see what to buy, the warehouse knows what to pick and finance knows what to invoice, without anyone passing details between separate tools.
Traditional ERP packages can be heavy and expensive to adapt, which is why some established businesses find them either too much or not quite right. The underlying idea is sound, though: one connected set of records. Tailored software can deliver that joined-up view, scaled to the size and shape of the business.