Accumulation is the collecting of products on a conveyor to move them as a group instead of individually. Accumulation occurs by halting the product on the conveyor allowing the products behind it to catch up and group together.
Accumulation is needed when you want to move a slug of products through at one time, typically seen when packaging multiple products in a single bundle. Or, if one part of the production cycle runs at a different rate. Or as a safe guard against a line shut down if a machine at the end of the line goes down. You can continue running and accumulate product before the downed machine instead of shutting down the entire line.