"Caging" has two meanings. First, it is a term used in direct mail operations to describe the process of opening, sorting, and coding donor mail and correspondence. The coding creates a "caging list" which is the organized data that is then given to the organization using the services of the direct mail contractor. That processed information is used to update the customer's lists.
The second meaning of "caging" is a form of voter suppression. It is a term that describes the process of challenging the validity of the voter registration of a voter so that they are required to prove they are registered before their vote will be counted. Wikipedia offers this description of the use of "caging" to suppress voter turnout:
Voters targeted by caging are often the most vulnerable: those who are unfamiliar with their rights under the law, and those who cannot spare the time, effort, and expense of proving that their registration is valid. Ultimately, caging works by dissuading a voter from casting a ballot, or by ensuring that they cast a provisional ballot, which is less likely to be counted.This form of voter suppression obviously works a lot better on people who do not routinely carry a variety of forms of ID and who find it difficult to get time off from work to take care of personal matters, so the Republicans tend to use it to reduce Democratic voter turnout.
With one type of caging, a political party sends registered mail to addresses of registered voters. If the mail is returned as undeliverable - because, for example, the voter refuses to sign for it, the voter isn't present for delivery, or the voter is homeless - the party uses that fact to challenge the registration, arguing that because the voter could not be reached at the address, the registration is fraudulent. It is this use of direct mail caging techniques to target voters which probably resulted in the application of the name to the political tactic.
On the day of the election, when the voter arrives at the poll and requests a ballot, an operative of the party challenges the validity of their registration.
While the challenge process is prescribed by law, the use of broad, partisan challenges is controversial. For example, in the United States Presidential Election of 2004, the Republican Party employed this process to challenge the validity of tens of thousands of voter registrations in contested states like Florida, Nevada, Ohio, and Wisconsin. The Republican Party argued that the challenges were necessary to combat widespread voter fraud. The Democratic Party countered that the challenges were tantamount to voter suppression, and further argued that the Republican Party had targeted voter registrations on the basis of the race of the voter, in violation of federal law.
I seriously doubt that Monica Goodling was using the term "caging" in its second meaning as she was testifying before the House Committee this morning.