Mysql locking

Hi all ,
I’m new to mysql locking !

I’ve 2 questions :

the 1st : I quoted this line from this tutorial :

If locks aren’t used, the items table can be modified by another user,

It seems to me that the user they r talking abut is a mysql user , not a visitor to the site !

What i mean is if a read lock is hold for some user , only this user can select , but all of my website visitors are with the same mysql user !!
I hope i understood it wrong !! I hope the word user refers to a visitors to the site not to a mysql user !!

Can some help me there please ??

The 2nd question : There is some point not clear for me , I programmed many PHP-mysql scripts befor knowing anything about locking , and no problem appeared ,

and another thing is how to use locking in mysql ? i didn’t understand if they mean that I should write the code to start and end the locking , or mysql manages this isssu itself …

Thanks for any trial to help …