What is the difference between a keyword and an argument in a Cisco IOS command?

A keyword is a fixed word the operating system already knows; an argument is a value you choose and supply yourself.

Every IOS command line is built from a command followed by keywords and/or arguments, and telling them apart matters because keywords can be tab-completed and discovered with ?, while arguments cannot — you simply have to know what value to type.

• Keyword — a specific parameter predefined in the operating system. It is always spelled the same way and IOS recognises it. In show ip protocols, both ip and protocols are keywords that narrow what show reports.
• Argument — a value or variable supplied by the user; it is not predefined, so IOS can't suggest it. In ping 192.168.10.5, the address 192.168.10.5 is the argument — you decide what it is.

Breaking down Switch>ping 192.168.10.5:

• Switch> = prompt (tells you the mode)
• ping = command (the action)
• 192.168.10.5 = argument (the user-chosen target)

Go deeper:

• Command-line interface — commands, options and arguments (Wikipedia) — the general grammar of keywords vs. user-supplied arguments that IOS follows.