What are biosignatures?

Biosignatures are

visual or chemical clues that signal the presence of life, past or present, in places where scientists won't have the luxury of doing sophisticated laboratory experiments. 

                                                                                              Michael Lemonick, 2014.

Those places include niche environments on Earth as well as possible habitats on other planets and moons of our solar system and on exoplanets and their moons.

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