These tell the CLI the specific details needed to perform an action. Some actions work without any additional details, but most require one or more parameters and accept additional optional parameters to perform its task. In fact, the CLI requires at least one special parameter named action (that tells what action to perform), without which CLI simply displays help with some example actions. Parameters do not have prefixes like actions and are often very specific to the app for the action.
Consider an example action getServerInfo that requires details about the app (or server) from which the information is retrieved. This is accomplished using the server parameter. The getServerInfo action also accepts optional parameters such as outputFormat, dateFormat, file, append, and encoding.