Primary Succession can be defined as the process of growth in an area that was previously uninhabited, barren, unoccupied and there was no initial vegetation. On the other hand, Secondary Succession can be defined as the growth of the community in such area which was previously occupied, inhabited and that has primary vegetation but got disturbed or damaged due to some external or internal factors.

Hi,

Primary Succession is one of two types of biological and ecological succession of  plant life, occurring in an environment in which new substrate devoid of vegetation and other organisms usually lacking soil, such as a lava flow or area left from retreated glacier, is deposited.

Secondary succession the series of community changes which take place on a previously colonized, but disturbed or damaged habitat. Examples include areas which have been cleared of existing vegetation (such as after tree-felling in a woodland) and destructive events such as fires.

Hope this helps.