A “lid” is what White House press secretaries use to indicate that there will be no news coming out of the White House that day. It can also be called a “Full Lid.”
The White House sometimes calls a lid when it does not want to release any information about a key topic. They call a lid to give notice to journalists that no questions will be answered.
Although the term has been around for decades, it was popularized by fictional Press Secretary C.J. Cregg on TV show The West Wing.
A presidential campaign can also call a lid when their candidate does not plan any more public events or appearances for the rest of the day.