Reapportionment is the redistribution of seats in the U.S. House of Representatives based on changes in population. These changes are recorded by the U.S. Census, which is conducted every 10 years.
In the House, federal law requires that there is always a total of 435 seats (with each seat representing one district). As states change population at different rates, the number of those 435 seats each one holds can go up or down—that is reapportionment.
Redistricting happens after reapportionment, so that each district has roughly the same number of people.