What is a biofilm?

A biofilm is

a community of microbes bound together in a viscous, gooey blob. 

                                                                                              Michael Lemonick, 2014.

Such biofilms are ubiquitous on Earth—likely growing in your shower and frequently found on other inorganic surfaces in wet environments as well as on organic surfaces including those of plants. If found beyond Earth (for example, in caves of Mars), biofilms would take on the role of exciting biosignatures for alien life and thrive—indicating that we, or at least “our” microbes, are not alone.

Reference
Michael D. Lemonick: The Hunt for Life Beyond Earth. National Geographic July 2014, 226 (1), 26-45.