First of all, do not worry. Your lovely ds is still your lovely ds, just as he was before you heard the diagnosis. 
Global development delay, really is just a description of what they see - just like you have described in your OP..... the expectation is that most dc tend to be up and walking around 18 months - you say he's not quite there at 22 months. That's fine - you can see he's going to get there, so in itself, that's not a major concern. Then you add in that he's not yet saying anything - again, by 22 months, you would generally be expecting quite a lot of single words, and some 'putting together' of words or short phrases, but he's not quite there yet.
So 'delay' indicates that he's not quite reaching the usual milestones at the expected time
'global' indicates that there isn't an issue with one area (say either speech or independence skills or physical, etc.) but that the delay is across all areas of development.
It really is very common in babies that were born prematurely, and that is why they continue to monitor him. However, the fact it is "just" a delay, means that they expect him to get there with all these things, just a bit later than the "average" child.
Having a diagnosis can make it easier to get some extra support though, at Nursery or maybe a referral to a speech therapist, for an assessment and some advice.