As an astronomer, can you explain Kessler Synfrome?
Not strctly astronomy.
But if you've seen Gravity, it's what (first) goes wrong there.
If you haven't, imagine two satellites (or other bits of debris, already) orbitting the Earth in different orbits (angle, mean distance, eccentricity, all can be vastly different) that at some point intersect. Deliberately or by chance. Smash! At orbital speeds (could be head on, so twice the instantaneous orbital speed, which is fast).
Now you've probably got a lot of fragments flying through space. Some may immediately de-orbit/escape orbit, without hitting anything, but many bits have a chance of intersecting with other orbitting items. Smash! Smash, smash, smash! SmashSmashSmashSmashSmash!!! Creating even more fragments, creating
more more fragments. Until the chances of anything else up there (or later being put up there) not being hit is pretty remote.
Eventually this cloud of debris will deplete (rationalise into mostly ordered Saturn-like rings, maybe, what doesn't get shoved towards re-entry or become a comet-tail spewing out from Earth into inter-planetary space). But if the thing ever starts to go all Syndromy it's likely to put a brake on future launches for a while. Thus there's a lot of interest in de-orbitting or graveyard-orbitting EoLed satellites, where possibly, and developing
methods for dealing with items where it wasn't possible.
HTH.