An adjournment motion is literally a motion: 'That this House (or sitting) do now adjourn'. In the Commons, however, debates on adjournment motions have covered almost every imaginable topic. This is because they have conventionally been used as a device to enable the House to discuss a particular issue in general terms and without needing to make a specific decision at the end.
Nowadays these general debates often take place on the motion: 'That this House has considered [a specific issue]'.
In the House of Lords, general debates can take place on a 'motion to take note' of a particular matter, or by tabling a Question for Short Debate.