Well before I say anything, you do need to know that the SATs are shite at the best of times but this year has been an absolute fiasco, so any SATs level your child is awarded (from the exam) isn't worth the paper it's written on. You should also have a Teacher Assessment level though, which is based on your dc's teacher's knowldege of them and their work all year.
And now I've said that.... The national average that the Govt would like every 14 years old to achieve at is Level 5. A good level 5 would indicate that, if the child works at the same level over the next two years, they are probably capabel of achieving a C grade or above. (But that's all variable of course).
At this key stage it goes up to a level 7 which the highest ability pupils would achieve. The lowest level for the SATs exams is a level 3 or 4, depending on whether it's maths, english or science. If they achieve less than this they are given an 'N' which doesn't count.
Ignore SATS results - they are a government tool and have no intrinsic educational value imho. They tell you squat, and upset the kids who then label themselves as thick if they don't get 5s and above, and set themselves up to fail their GCSEs. That's why I loathe them...I've seen the effects.
SATS are only for 3 subjects anyway. If they'd had them when I was at school, I'd have ploughed maths and science, yet here I sit, with an upper second class degree and a PGCE, waiting to see if I've been accepted on an MA course. SATS are a waste of time and effort and an unwarranted stress on both staff and students.