After you have enabled the Security Admin, going forward you will be prompted to enter your access key (that you specified when you enabled the option).
From the moment you supply the access key to 'unlock' admin access to the plugin, a countdown timer will begin after which you will be locked-out again.
Any further access to the plugin admin will prompt you to enter your access key.
So the Security Admin Timeout is how long that countdown timer will last. Default: 30 minutes.
Note: This does not apply until you enter the access key again.
For more information on Security Admin, read the blog article here.