Linked records are required to use three field types: lookup, count, and rollup.

• A lookup field returns the value(s) for a specific field from a linked record (or records). You can use this to bring information from one table into another table without duplicating it, so that your information will always be up-to-date in all tables.
• A count field returns the number of linked records in a linked record field.
• A rollup field is a bit like a lookup, but instead of just returning the value(s) from a field in a linked record, it also performs a calculation on those values—like summing together all the values from the linked records, or returning only the maximum value from the linked records.