A transient prefix command is invoked like any other command by pressing the key that is bound to that command. The main difference to other commands is that a transient prefix command activates a transient keymap, which temporarily binds the transient’s infix and suffix commands. Bindings from other keymaps may, or may not, be disabled while the transient state is in effect.
There are two kinds of commands that are available after invoking a transient prefix command; infix and suffix commands. Infix commands set some value (which is then shown in a popup buffer), without leaving the transient. Suffix commands on the other hand usually quit the transient and they may use the values set by the infix commands, i.e. the infix arguments.
Instead of setting arguments to be used by a suffix command, infix commands may also set some value by side-effect, e.g. by setting the value of some variable.