What is the use of scheduler class in Salesforce?

Scheduler class is basically an apex class or a batch class which you want to run it at certain point of time.

Suppose you want to send an mass email to all your contacts on the end of every month. In that case you can schedule a class which will automatically send the emails on every month end.

For scheduling an apex class, you can do the following

From Setup, enter Apex Classes in the Quick Find box, select Apex Classes, and then click Schedule Apex.
Specify the name of a class that you want to schedule.
Specify how often the Apex class is to run.For Weekly—specify one or more days of the week the job is to run (such as Monday and Wednesday).
For Monthly—specify either the date the job is to run or the day (such as the second Saturday of every month.)