Travel/Photo lid
It’s a term that is used in the current White House and in past white houses (i have a 1996 reference) in Kenneth T. Walsh’s book Feeding the beast.
Per this politcco story it’s been used in the Obama White House
In the context of White House press briefings, a "photo lid" or "lid" signifies that the day's public events for the President are concluded, and there will be no further photo opportunities or appearances for the remainder of the day. It essentially means the press pool is dismissed, and the President is not expected to be seen by the public or the press. A lid can be "lifted" if circumstances change, but this is not common.
When the Press Secretary signals to the White House press that there is no more news that the White House will be announcing, that’s called the ‘lid.’ A “full lid” means no more for the rest of the day. In other words, the reporters can go home.
There are other lids - like a ‘lunch lid’ which actually means the reporters have an hour and the White House promises not to announce anything so they can each lunch.
Journalists have complained of instances in which the White House declared a “full lid,” meaning no more scheduled releases or travel for the day, then later puts out a newsworthy release such as a readout of a presidential call with a world leader. A White House official told POLITICO: “We vowed to ensure that full lid violations were rare.”
Obviously as the story indicates the ‘lid’ is only as good as world events can be held back for a day. Sometimes the WH does have to announce news after the lid, and reporters often don’t like it.
