The congressional breakdown

The U.S. House of Representatives has a total of 435 seats (the result of a federal law from 1911).
Each seat represents one district throughout the U.S., and each district has to contain roughly the same number of people so that, in theory at least, everyone is equally represented.
Currently, the U.S. population is at a bit more than 311 million. So, divide that by 435, and it comes to roughly 715 thousand people in each district throughout the country.

California, with 53 U.S. congressional districts, is the most populous state in the nation and therefore has the most districts (and the most seats in Congress).
So if your district has increased in size over the last decade by, say, 100,000 people, then the geographic lines of that district need to be withdrawn to make sure it doesn’t have more people than any other district.

assem2011mapfinal — 2011 State Congressional Districts_California Citizens Redistricting Commission

Is it just congressional lines that get redrawn?

Nope! There are a whole bunch of state and local government jurisdictions that need to be redrawn too. In California, this includes State Senate, Assembly, and Board of Equalization, all of which have their own separate districts.That means that every ten years, California has to come up with four completely redrawn district maps. You can see all the final versions here.

There are also the local districts – including county, city, school district, community college district or special districts – which also get redrawn, but that process is controlled locally.

Resources

apportionmentapportionment

Redistricting Revealed: The Basics