Goto transaction SM36 and click on "standard jobs" push button. This displays standard jobs screen. Here select all the jobs and click on "default scheduling" push button to schedule all of them as per their default schedule.
Incase you would like to change the default schedule for each job, it can also be done by selecting each job and defining its start date/time and periodicity in the same screen.
Need of scheduling basis standard jobs.
Standard jobs are the jobs that should run regularly in the SAP system. These jobs will perform housekeeping like deleting old spool requests (thus avoiding spool overflow), deleting old background jobs/logs/updates/batch input sessions/ABAP short dumps, collecting operating system/database level statistics (used for workload reporting) etc