From the maintainer's site:

spamhaus_drop (Don't Route Or Peer) and EDROP are advisory "drop all traffic" lists, consisting of netblocks that are "hijacked" or leased by professional spam or cyber-crime operations (used for dissemination of malware, trojan downloaders, botnet controllers). The spamhaus_drop and EDROP lists are a tiny subset of the SBL, designed for use by firewalls and routing equipment to filter out the malicious traffic from these netblocks.

EDROP is an extension of the spamhaus_drop list that includes suballocated netblocks controlled by spammers or cyber criminals. EDROP is meant to be used in addition to the direct allocations on the spamhaus_drop list.

When implemented at a network or ISP's 'core routers', spamhaus_drop and EDROP will help protect the network's users from spamming, scanning, harvesting, DNS-hijacking and DDoS attacks originating on rogue netblocks.

Spamhaus strongly encourages the use of spamhaus_drop and EDROP by tier-1s and backbones.

There is also a FAQ for the EDROP list here.