Hi abadoo:
MN has an explanation of National Curriculum Levels (NC Levels - like NC L4b) and how your child should notionally progress through them here: www.mumsnet.com/learning/assessment/introduction
In England primary schools are divided between INFANTS and JUNIORS - sometimes these are separate schools entirely:
INFANTS = Reception (sometimes Year R) and Key Stage 1 (KS1: Years 1 & 2)
Year R (ages 4/5) is under the Early Years Foundation Stage (EYFS) - info here: www.gov.uk/early-years-foundation-stage
KS1 (year 1 - age 5/6 and year 2 age 6/7) are under the national curriculum. If your child is starting Sept 2014 - they will be taught under the new national curriculum - info here: www.gov.uk/government/collections/national-curriculum - if you scroll down a bit you can see the programme of study for different areas of the curriculum - these documents break down what is taught by individual years in KS1 & KS2 (years 3 - 6).
JUNIORS: Key Stage 2 (KS2 = Years 3 - 6 - so Y3 is ages 7/8, Y4 is 8/9, Y5 is 9/10 and Y6 is 10/11).
Given your DC will start Reception next school year (SEPT 2014) this will also be under the new national curriculum (see link above).
The floor standard (the bare minimum the government expects) is that 65% of all primary pupils in a school will leave Y6 with a score of NC Level 4 combined in both English & Maths on KS2 SATs (taken traditionally over one week in May). Notionally to be 'secondary ready' the government feels a child must be solidly working within NC Level 4 - which is described as NC L4b.
As explained above by purpleroses - the NC Levels are divided into 3 basic sub-levels:
starting c - then b - then a.
So for example a child working NC L3c is working just into NC Level 3, but not strongly working at Level 3.
A child working NC Level 3b is working well within NC Level 3 - but possibly not showing much sign of working beyond NC Level 3.
A child working NC Level 3a is working well within NC Level 3, possibly finding much of the work at this level fairly easy to cope with and showing signs of working toward NC Level 4.
Going back to your original question: What does this mean practically? How important is it compared to the general achievement result?
I suppose the way of thinking about it is the general achievement result will be the combined results for KS2 SATs (so Y6 SATs) in English (Reading & Writing) & Maths - so this is asking whether overall the kids are working at Level 4+ (i.e. notionally able to engage with secondary curriculum from the start). Now they should also show the results in terms of NC L5+ for last year's scores (2013).
So if in theory 50% of the school score NC L5+ and 90% score NC L4+ - that's telling you that 9 out of 10 kids at this school achieve the expected target (government floor target of NC L4) in the combined results of writing/ reading/ maths and 5 out of 10 pupils exceed that.
Individual subject scores: Spelling, punctuation and grammar (SPAG)
Science
Reading
Writing
Maths
will of course vary for each child - as children may have particular strengths in certain areas or more interest for one thing than another - so in theory a child could have scores that might range of NC L3 in Maths (a child who doesn't like/ get numbers) to solid NC L5s in science & English subjects. Overall they would fail to make the government floor standard because of the NC L3 in Maths.
I think the way to judge a school is to see if consistently over a number of years (say 5-6) they're getting a large proportion of pupils (70%+) to NC L4 or better in combined English/ Maths results. - You can find this info in two places: For 2012 & 2013 SATs (so may of those years): www.education.gov.uk/schools/performance/ - just enter school name or postcode. For data prior to 2012 - on the left hand side of this webpage you'll see Performance Tables 1994-2012 - select this and you can search the school's results going back a number of years.
I think seeing a nice, steady and consistent result or one that is improving year on year says a lot about the stability of delivery of a strong curriculum that's well taught. If there are blips (so a particularly bad year) what you want to see is whether this is a one-off - not the start of years of decline (as in our school which pre 2006 always had 90% L4 on KS2 SATs and then dipped to 80%, then 70% and recently 60% - the trend is strongly and steadily downward here I fear).
HTH