I sort of agree with the OP - it's easy enough to google this stuff - but I also think the levels are needlessly confusing and, worse, to some extent meaningless. You get obsessed with them and then realise it's pointless.
One of the confusing things is that the number bit of it starts at 1 and goes up 2, 3, etc. But the lettering goes the other way, so that a is higher than c.
And they don't seem to progress in an orderly fashion either, iywswim. So when children do SATs in year 2, the average is something like 2b, while the highest score will usually be 3a. But then over the next four years, they only progress a couple of levels, so that the average child in year 6 will be on 4b, and the brighter kids on at most 5a.
It's equally confusing when your child gets to secondary. If you go to the directgov explanation of how this works, the average 14-year old is supposed to be getting something like 5a/6c. Yet there'll be plenty of kids already at 5a level in year 6. Is there really such a gap between "bright" and "average" that the average 14-year old is achieving the same as the brighter 11 year olds?
Furthermore, most parents find that their kids drop a couple of sub-levels in their first year at secondary school because the primary school has been artificially pumping up the grades to look good in league tables. So if you took any notice of this stuff, you could get terribly anxious about it.