Prior to using this option, please think about making the user inactive first. Inactive users may be brought back, while deleted users will be gone for good.
Another point of caution when deleting a user - you should think about the System Account implications before you commit a delete. One important question to ask is: If it is necessary, have I transitioned this user's scope to someone else?
An admin may have his account manually locked or auto-locked. An autolock occurs should there be 3 unsuccessful login attempts. Should this occur, the system will lock the account for a 10 minute period at which point it will unlock and be available for login.
In the case of a manual lock an admin would need to manually toggle that setting to unlock.
Please find a writeup of how to manage scope here.
Permissions tell the system what an Admin user is authorized to do. If you are unsure of what a permission means you may find more information by doing a mouseover of the blue information icons.