In primary succession, the rocks that are formed newly is colonized. For the first time the process is done by living things. In secondary succession firstly an area is disturbed, which was previously occupied by living things. Then it is recolonised following the disturbance.

Hello shibam
In primary succession ,newly exposed or newly formed rocks is colonized by living things for the first time.
In secondary succession ,an area that was previously occupied by living things is disturbed ,then re- colonized following the disturbance
Hope this help you!