Elections & terms
apportionment
The once-a-decade division of the House's 435 seats among the states by census population. Redistricting is the states then drawing the district lines.
In full
After each census, the 435 House seats are reapportioned among the states by a fixed mathematical formula; every state gets at least one. States then redraw their congressional district lines, redistricting, subject to equal-population and voting-rights constraints. When lines are drawn to entrench a party or incumbent, that is gerrymandering, a practice as old as the republic and still fought over district by district.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.