Like many things in the game, spore buds do not exist in real life. They are small black and green flower-like buds that can grow seemingly anywhere. They apparently do nothing all day besides releasing (presumably) non-toxic spores into the air.
Spore buds are mysterious plants which serve no other purpose than to provide depth to the metro universe and direction in the game. The buds do not harm the player in any way but they do react to Artyom's presence. In some cases, shining a light at or walking nearby will cause them to close or open up (suggesting some kind of nervous or electrical system) and then slowly pulse again.
The developers of the game use the bud's bio-luminescence as a strategic marker indicating which direction the player should go. Such examples of which are when the plants illuminate some key platforms when navigating the underside of the railway line on Front Line, as well as the pipe entrance to the Nazi camp in the same area. They also highlight the appropriate path through the pipes once Artyom is thrown off the rail cart further in the level Depot.