An autoblock is, as the name suggests, an automatic block of an IP address set by the MediaWiki software. Autoblocks are the result of an attempt to edit Wiktionary from an IP address recently used by a blocked user.
Each time a user edits Wiktionary, the IP address used to connect to the site is recorded by the Mediawiki software that powers Wiktionary. A log of IP addresses used by every user is kept privately, accessible only by users with checkuser access and to the Mediawiki software. When a blocked user attempts to edit the site, the IP from which they are editing is "autoblocked," so that they may not make the same edit anonymously or under a different user name.
Many times, when a user that edits from an ISP with a dynamic IP pool, such as AOL, is blocked, all the IP addresses recently used by that user will be autoblocked. In most cases, it is not the blocked user, but an innocent, uninvolved user who happens to share the same ISP, who is autoblocked. This type of autoblock is referred to as collateral damage.
There is an internal autoblock expiry time variable, which is set to 24 hours, meaning that autoblocks last for 24 hours at most. However, in the case of dynamic IP pools, this may affect hundreds of users before the block expires. In the case of an indefinite block, autoblocks may continue to be set by the software weeks or months after the initial block has been set. (It is not uncommon to find autoblocks listed on Special:Ipblocklist where the initial indefinite block was issued in 2003 or 2004.) Users who find themselves autoblocked accidentally should contact an administrator and request an unblock.
It is helpful for all involved to remain patient and remember that it is the software, not the admin who is unfortunate enough to have his or her name appear on the block, who is responsible. Equally important is for admins to check Special:Ipblocklist regularly, and unblock all autoblocks from a particular user if more than two autoblocks are set in rapid succession. (If more than two IPs are autoblocked within seconds/minutes of each other, it is a good indication that it is a dynamic IP pool and the blocks are collateral damage.)
Autoblocked users should email the admin before posting angry messages as 1) it isn't the admins fault, and 2) it can quickly be taken care of without the need for hurt feelings. Make sure to note that it is an autoblock, as the admin may mistake your ability to post to his or her talk with being unblocked. It is imperative that you include the IP address that is reported in the block text, as the admin has no way of determining which IP is autoblocking you.