What does it means to reapportion seats in the house of representatives?

Answer 1

reapportion means to redistribute

every ten years the house seats are reapportioned when the united states counts its population.

Answer 2

Reapportionment is the process by which the seats of the House of Representatives are redistributed among the states based on changes in population according to the Census which is conducted every 10 years. While every state has two Senate seats regardless of population, the seats in the House of Representatives are distributed among the states based on population. Thus states which gain population can gain house seats, and states which lose population can lose seats.