The cat (Felis catus) is a domestic species of a small carnivorous mammal.

And is also a standard Unix utility, yay!

Cat is used to read files sequentially, writing them to standard output.

Open your terminal; use touch to create a file:
$ touch file

$ echo "Hello World" >> file

Prints "Hello world"
$ cat file

Copy the file contents:

$ touch file2

$ cat file >> file2

Prints "Hello world"
$ cat file2

Concatenate multiple files:
$ touch file3

$ cat file file2 >> file3