I believe the following quote is clear:
Limiting WIP is the cornerstone of Kanban. Limiting work-in-progress
implies that a pull system is implemented. Put limits on columns in
which work is being performed. The critical elements are that
work-in-progress at each state in the workflow is limited and that new
work is “pulled” into the next step when there is available capacity
within the local WIP limit.
By limiting how much unfinished work is in process, you can reduce the
time it takes an item to travel through the Kanban system. You can
also avoid problems caused by task switching and reduce the need to
constantly re-prioritize items.
In case you have a particular question here - let's discuss.