What is garbling in the context of radar?
Garbling occurs when two aircraft are close enough that their replies overlap, as there is no reliable way to sort them out. Usually this occurs when the aircraft are passing above each other as the radar scans the entire vertical column at once, only rotating horizontally.

The problem is that the controller might not be able to check the aircraft are indeed flying their correct assigned altitudes exactly at the moment when it matters most when they are passing above each other.

Quick Googling reveals e.g. this article with some diagrams of the overlapping signals.

A related problem is “fruit”, which is the squawks triggered by other things than the radar beam (e.g. the interrogations from T-CAS on other aircraft) being mistaken for ones that were, and therefore making the aircraft appear at other place than it really is.

Both issues are mitigated by ADS-B, because there the aircraft broadcasts at semi-random points and includes it's own position, so while the messages may still conflict (like in other transmission protocols), the timing is designed to make it unlikely to happen for several following messages.