"tail -3 foo | od -bc" means to take the last three lines from "foo" and feed it to the "od" command with "b" and "c" options.

I'll presume that the first part is either clear or is reasonably easy to look up; "od" is the weird part. This is named for "octal dump" (option b) but I'm using it here to also get the character names (option c). In particular, to reveal the non-printing characters.