The CELF-Preschool is a standardised assessment tool which measures a child's performance on the test against other children of their age. Each subtest yields a 'standard score'. The average standard score for each subtest is between 7-13. A score of 3, for example, would indicate that the child is having significant difficulties in comparison to other children of the same age. A score of 10 would suggest they are smack-bang in the middle of average. A score of 14 would suggest they are above average.
In addition to giving this numerical score, the test allows an SLT to analyse different aspects of language to see which areas are strengths and which areas your child has particular difficulty with, as children with language disorders do not present with identical profiles. For example, some children's errors may be more around grammatical understanding rather than vocabulary, others may have significant difficulties with vocabulary, others may have poor auditory memory which is underlying their poor performance. Analysing the pattern of errors can also give the SLT info on what would be most appropriate areas to target in therapy.
Understanding Sentence Structure, Following Concepts and Directions, Understanding Basic concepts:
These subtests are all what we call 'receptive language' subtests - they look at different aspects of comprehension. It sounds as though your SALT is trying establish what your child's comprehension of language is like in comparision to other children his age. That's not to say his difficulties don't lie in other areas - however the therapist will want to establish if there is a discrepancy between what he can understand versus what he can say, or if his difficulties with what he can say are as a result of underlying difficulties with understanding. Some children can be good at using environmental cues to support their understanding, but when tested, gaps in language understanding can be evident. I'm not saying this is the case with your son, of course, but the therapist will be wanting to rule comprehension difficulties out.
'Understanding sentence structure' looks at a child's understanding of grammatical elements. The therapist says a sentence and the child points to the picture that matches the sentence. Sentence forms include different verb tenses (going to/-ed/passive tense), prepositions, indirect objects (e.g. mum showed the dog the cat) to name a few.
'Following concepts and directions' looks at a child's ability to follow instructions containing a range of concepts as well as their ability to follow instructions of differing length (e.g. one-part, two-part and three-part instructions). A child with auditory memory difficulties will have difficulties following longer instructions. Concepts are slightly more abstract and more linguistic in nature than basic concepts and include things like between / next to / before / after / except / unless etc. The therapist gives an instruction, and using a visual stimulus, the child is expected to carry out the instruction.
'Understanding basic concepts' looks at how well a child understands, well, basic concepts. The instructions are kept very short (one-part) as the focus is on the child's understanding of the concepts. Basic concepts are easier than the concepts contained in 'following concepts and directions'. Examples of basic concepts are in, tall, large, same, different, long, short, empty, full etc. Again, the therapist says a sentence and the child points to the picture that matches.
Sorry - long post - but hope this helps.