What does cat function do?

cat is useful for producing output in user-defined functions. It converts its arguments to character vectors, concatenates them to a single character vector, appends the given sep = string(s) to each element and then outputs them.

What does cat grep do?

The grep command searches one or more input files for lines containing a match to a specified pattern. By default, grep prints the matching lines. Grep searches the named input FILEs (or standard input if no files are named, or the file name – is given) for lines containing a match to the given PATTERN.