CPU monitoring is essential to understand fully performance. Steal time, or in our case percentage, is a value raised by the kernel to express the amount of time the CPU was unavailable for the VMs tasks. This time can be stolen by the hypervisor itself or by other tenants. In few words, CPU steal represents the amount of time you can't use your CPU.
This is one the principal metric to measure VM density and/or noisy neighbors. Bare metals and dedicated CPUs are supposed to raise a constant null value, while VMs, especially the smaller, tend to output values from 0 up to 90%.