To aggregate is to join things together, so from an IFRS perspective we could join similar balances together on the financial statements, such as some non-current assets.
Something is material if it is large or significant. So from an IFRS perspective we can join together (aggregae) two balances, if one of the balances is not material (large). An example would be if we had immaterial intangible balances we could aggregate them with the PPE balances.