Biological Data is obtained from the analysis of certain organisms, usually those captured by Planet Landers on a planet or moon's surface. While Commander Hayes states that such a "search for abstract knowledge" is of little worth given the current conflict, the biological information is of are of value to the Melnorme, who will trade two Interstar Credits for every unit of biological data.
Apparently, there are ways to obtain biological data other than via the landers. The Umgah, biological fanatics themselves, give The Captain five hundred units of biological data as part of a reward for freeing them from the psychic control of the neo-Dnyarri. The Slylandro also traded biological data from Source to the Melnorme for 10,000 Credits, which they in turn used to purchase their first Probe.