What is a lockbox?

On the instruction it says I need to send my document to a uscis lockbox nearby my area but idk what it is ...... Is there one nearby Seattle Washington????

Here you go:
http://www.uscis.gov/portal/site/usc...0045f3d6a1RCRD

http://dreamact.info/forum/showthread.php?t=29368
that should help

and i can use any delivery service to send right? I am going to use USPS first priority mail. is this acceptable?