Virtualization and resources
Overselling
A provider sells more resources than the server has, betting that not all customers use them at once.
Most VPSes sit idle, so providers oversell cores: a server with 64 threads may host machines with a hundred vCPUs in total. Within reason this is normal and makes VPS cheaper.
It is bad when overselling is heavy: the server slows down at peak hours even though your load has not changed. The signs are unstable CPU and disk speed. The cure is dedicated cores and reading provider reviews.