This column shows the percentage of the cpu that each process is currently consuming. By default, top will sort this column of the output.
Some versions of Unix will track cpu percentages in the kernel, as the figure is used in the calculation of a process's priority. On those versions, top will use the figure as calculated by the kernel. Other versions of Unix do not perform this calculation, and top must determine the percentage explicity by monitoring the changes in cpu time.
On most multiprocessor machines, the number displayed in this column is a percentage of the total available cpu capacity. Therefore, a single threaded process running on a four processor system will never use more than 25% of the available cpu cycles.