cat is a well known command to concatenate the content of multiple files. Example: cat file1 file2 file3
But there are other use cases. cat offers a nice way to print out multi line strings. It is even possible to include variables into the string, which feels a little bit like using a templating language.
NAME=ADMIN@COMPANY.COM; cat <<EOF Hello $LOGNAME, please be aware. This system will be under maintenance soon. Have a good day. Sincerely $NAME EOF
For more info on the <<EOF visit this SO-Thread