Aliases let you create aliases for commands. They are simply added at command regisration time by using either of the following methods:

CommandAPI.getInstance().register(String, String[], LinkedHashMap, CommandExecutor);
CommandAPI.getInstance().register(String, CommandPermission, String[], LinkedHashMap, CommandExecutor);

The String[] represents a list of aliases which can be used to execute a command.

Example - Using aliases for /gamemode

LinkedHashMap<String, Argument> arguments = new LinkedHashMap<>();
//populate arguments here

CommandAPI.getInstance().register("gamemode", new String[] {"gm"}, arguments, (sender, args) -> {
    //Handle gamemode command here