What does "if and when" mean, and is it the same as "when and if"?

The idiom "If and when X, (consequence)" has the meaning "if X happens, then when X happens, (consequence)". In other words, the "(consequence)" will not happen unless X happens, and will not happen before X happens.

It is generally used when talking about not making unnecessary preparations for an event that may not occur:

We'll come up with a zombie attack mitigation plan if and when we're attacked.

Some people might complain that "if and when" is redundant, because the "if" carries with it a sense of "when", in that you don't know whether X is going to happen until it actually does. But it's sometimes possible to know to some degree of certainty that something is going to happen, before it actually occurs - and in fact this is often the situation where the phrase is used:

We'll open fire on them if and when they open fire on us.

Simply saying "if they open fire on us" can invite the response "but we know they're going to, so we should fire now"; the when emphasises the speakers view that no pre-emptive action should be taken.

As to the difference between "if and when" and "when and if", the basic meaning is identical (as others have noted), but there is possibly a slight difference of nuance: the standard phrase is "if and when X", but inverting it to "when and if X" emphasises the if part, thereby putting more emphasis on the fact that it is uncertain whether X will actually occur.

It's an idiom. You're not meant to analyze its component parts. That's what "idiom" means. ☺

If and when and when and if denote pretty much the same thing: something that's not as sure as "when", but more certain than "if".¹ (Or, more cynically, it's just an overly-wordy way to say "if".)

I think there's a very slight emphasis on the first word: if and when is a teeny-tiny bit more conditional than when and if.

¹ I did find one dictionary that supplied "if and when" as the US equivalent of UK as and when, which they define as "at the time that something happens". I'm not sure I agree with this: as a US English speaker, I perceive a slight but definite difference in meaning between "as and when" and "if and when".