The meaning is actually relatively literal. The BBC summarises it here:
Parliament retains the right to hold debates in private and any MP can propose that members of the public and press be removed from the galleries at any time.
If such a proposal is made, the Commons Speaker must put the motion "That this House sit in private" to a vote without debate.
But the procedure is seldom successful - attempts to exclude members of the public are usually deployed as a device to disrupt or delay the House's proceedings.
In case it's not clear, 'galleries' should be interpreted as 'viewing areas into the debate hall'.