Final answer:
Marking up a bill refers to the process of amending it, which involves making changes, additions, or deletions to the initial draft during committee discussions before it advances in the legislative process.
Step-by-step explanation:
When referring to the process of a bill being worked on in the legislature, saying a bill is being marked up is just another way to say it is being amended. This stage of the legislative process involves a committee going through the bill and making changes, additions, or deletions, which are essentially amendments to the initial draft of the bill. Once the committee has finished marking up the bill, it can move forward in the legislative process, either to be voted on by the full chamber or to be sent to another committee for further consideration.