It is short for concatenate, which is to join things together. You can give it multiple inputs and it will output each one directly following the previous. It so happens to also work with just one input.

It is to use along with split. e.g.

1. You take a single large file, say 16GB
2. Use split to break it into multiple files of 4GB
3. Now you can transfer it to a FAT32 Removable Flash Drive and transfer it to whatever other computer that doesn't have Ethernet.
4. Here, you can use cat to combine all files into the original file. (preferably accompanied by a checksum)