What is going on is that if an MP tries to ask a specific question to the Prime Minister, the PM will avoid answering the question by just saying it's the responsibility of some other Minister. This is a truthful answer and is permissible as the PM has no specific responsibilities of his own. So to avoid Prime Minister's Questions being a total waste of time, the practice developed of asking him what his engagements are for today. This is taken as an "open question" and the standard answer is "This morning I had meetings with Ministerial colleagues and others. In addition to my duties in this House, I will be having further such meetings later today."
So that when the MP stands up again to ask his supplementary question, it can be about anything, and the PM can't avoid it. A supplementary is always allowed in questions to Ministers. And the Speaker can pick an MP who didn't put down in advance to ask a supplementary question if he wants to.
So what you get is that the entire list of questions put down in advance will almost certainly all be asking about the PM's engagements, and the second or later time this is asked, you can see that a sensible answer is "I refer the honourable gentleman to the answer I gave some moments ago". It has now evolved to the point where they have dropped this fiction that an MP actually DOES want to know what's in his diary and just save wasting time by getting straight to the MP's real question, so they don't say that any more.
Another tactic that is sometimes used will be like the question I remember being asked to Margaret Thatcher "When does the Prime Minister next plan to visit Pratt's Bottom?" (I'm not joking - Pratt's Bottom is in Kent.) Of course she answered that she has no plans to go there, and the supplementary was "Well if she did she would find that the people of Pratt's Bottom are worried about..." What often then happens is an MP who is picked on by the Speaker to ask the next supplementary forgets the original question wasn't an engagements question, and gets ruled out of order for not mentioning Pratt's Bottom.
Edit - as other answerers bring it up, the "honourable gentleman/lady", or "right honourable gentleman/lady" if the person is a member of the Privy Council (anyone who's ever been a Cabinet Minister) is not sarcasm, it's just Parliamentary protocol. It is the rules that MPs can't refer to each other by name and it's supposed to tone things down when they get heated by preventing them from being rude about each other. Variations on this are that they will refer to members of the same party as "my honourable friend", MPs who are lawyers are "the honourable and learned gentleman", and MPs who have served in the armed forces are "the honourable and gallant gentleman".