From the maintainer site:

A bogon prefix is a route that should never appear in the Internet routing table. A packet routed over the public Internet (not including over VPNs or other tunnels) should never have a source address in a bogon range. These are commonly found as the source addresses of DDoS attacks.

Bogons are defined as Martians (private and reserved addresses defined by RFC 1918, RFC 5735, and RFC 6598) and netblocks that have not been allocated to a regional internet registry (RIR) by the Internet Assigned Numbers Authority.

It is important to realize that the bogon list is NOT a static list.

How much does it help to filter the bogons? In one study conducted by Rob Thomas of a frequently attacked site, fully 60% of the naughty packets were obvious bogons (e.g. 127.1.2.3, 0.5.4.3, etc.).

For more information please refer to the maintainer site.