Well, you are in the right place, this is what they are:
Chewing gum, yup that's right!
It's chewing gum that people spit out anywhere they please and as the time passes and it gets stepped on so many times it becomes this black flat spot (as on the picture above) that is really hard to remove!
Effects of the spit chewing gum:
It makes the place look dirty and less appealing.