CPU units are CPU priority. The CPU priority comes into play when there are multiple VPS accounts needing more than the total amount of CPU processing power available at any given moment.
An example would be two hypothetical VPS accounts, VPS-A with 800 CPU units and VPS-B with 400 CPU units. If both VPS accounts request 100 percent of the CPU at the same time, VPS-A will get twice the CPU time that VPS-B gets, since 800 is twice as large as 400.
A VPS could still get 100% of the CPU if no other VPS accounts on the hardware node needed it, but that percentage would drop down as soon as another VPS needed CPU time.