Kessler Syndrome is a theoretical scenario in which the density of objects in low Earth orbit (LEO) is high enough that collisions between objects could cause a cascade in which each collision generates space debris that increases the likelihood of further collisions.
It is sometimes described in what sound like sensationalistic terms as something that could easily get so bad that it would straight-up prevent any future space-flight. I find myself skeptical of this, given that it would take a LOT of debris (which also must be large enough to pierce at least a trivial layer of armor) to provide unmanageable risk for a rocket that is passing through the LEO debris shell to a higher orbit (or to Earth orbit escape) in a matter of minutes.
Currently, the highest density of space debris is in a shell that starts at an altitude high enough that decay due to atmospheric drag is too slow, and ends at the upper edge of "lower" earth orbit (as far fewer satellites are launched to higher altitudes). This isn't a particularly large shell, and it seems like what makes satellites impractical over a period of months is very different from what seriously imperils a rocket that passes through in a few minutes (and might still have its payload fairing in place, since the majority of dV is expended in horizontal thrust, not vertical.)
