You would use that expression to strongly emphasise something as refecting/representing/showing a particular point. It's unusual use, arguably colloquial, but it is correct.
For example, one person might say: look at the new clothes I bought (perhaps suggesting 3-4 items - the context being, the person is happy about the amount/quality of clothes they have). A second person takes the first to her closet, opens it up and shows her a hundred new items of clothing. To rebut the first person's idea and to emphasise what the second person thinks, the second person says "
this is clothes.
There are lots of expressions which start "this is ..." as a strongly emphatic form.