I can guess, that they have something in common with wards. But what is it? Can enemies see me thanks to it?
They are places where wards were placed that have since timed out. The ones with red over them were placed by the other team. Enemies cannot see you because of it, but can tell where wards were placed for a short time afterwards.
Note: the colored orb (blue/red) above the stone will fade over time before disappearing, giving you a general idea of how long ago it was warded.
source- personal experience. Also pictured below, a bunch of ward stones.
This is called "ward rubble" or "ward debris" and was added in Patch 5.22.
Upon death or expiration, wards leave behind Rubble. Rubble is team colored, lasts up to
54 minutes, and appears as circles where the ward was when it was active.
It used to be 5 minutes, but this was changed in Patch 6.2 to 4 minutes.
- Brightly glows in the first 10% of its lifespan!
- Stops emitting flames after 30% of its lifespan has passed!
- Particle visibly changes and debris shrinks after 60% of its lifespan has passed!
These features allow you to see and keep track of where the each team wards and how long ago the wards expired. This is helpful, as you can see a newly expired ward and know that the enemy laner(s) won't see a gank coming.