DB2 allows to encrypt data both in storage and in transit. Securing stored data makes sure that even if someone can access the disk, the actual data can not be read. Encrypting data in transit prevents sniffing, i.e., someone trying to listen to the communication channels and spy on the data (Footnote: this is not entirely true). There exist different options in DB2 for securing data on disk and during communication. SSL, the Secure Socket Layer, is a commonly used technology to encrypt communication, e.g., for Web access with the HTTPS protocol.
A new article has been published on developerWorks titled "Secure Sockets Layer (SSL) support in DB2 for Linux, UNIX, and Windows" that provides step by step instructions on how to setup SSL for use with DB2. If you haven't looked into that topic before, this is a good starter.