Akin to unsolicited junk mail. The verb refers to the sending of unwanted email to many people at once. Originates from a Monty Python sketch in which "spam" is said an excessive amount of times.

To indiscriminately post information -- often of an irrelevant or commercial nature -- to multiple newsgroups or e-mail recipients. Derived from an old Monte Python sketch, "Eggs, spam, spam, spam, bacon and spam."

Basically sending E-mails to people who in no way asked you to send that information - normally done in huge numbers to promote a product.