Film for lo-fi cameras comes in rolls. It is a long strip of plastic, coated on one side with a light sensitive emulsion.
An important thing to know is that film is light sensitive until processed at a lab. Therefore...
Do not open your camera unless you are loading or unloading it!
I realize that this may seem obvious, but many people have never used film and have never seen it used.
The film for most lo-fi cameras come in a few different sizes. The most common sizes are 35mm and 120. I should note that the 120 in 120 film is a name, not 120mm. The actual size is closer to 70mm. Kodak invented this size back in the early 1900s, and it is just a number designation.
When asking questions about 120 film, remember to say "120" and not "120mm".
For a full explanation of what film is and how it works, check out the wikipedia.