Primary succession is colonization of new land (e.g. created by lava flow or glacial retreat) which initially has no soil and is devoid of life. Such species are termed "pioneer species".
Secondary succession is change in the composition of species in an already established ecosystem, which has already hosted vegetation. This is the result of a sudden or gradual change in environmental conditions. Sudden changes might be wildfire, extreme weather events, or various human activities such as logging. More gradual changes might be climate change, or the natural development of an ecosystem after pioneer species start to alter conditions.
For most parts of the world, undisturbed land will trend towards a particular balance of species which remains stable until it is disturbed. This stable ecosystem has undergone late succession and is called the "climax community". Ecosystems that are part way towards the climax community are in "mid succession".
As an illustration, a pasture field or a crop field in a temperate climate. The field is continually maintained by the activity of humans and their livestock. The predominant vegetation is grass or other monoculture, and species diversity is low. If the livestock are removed and the land ceases to be disturbed, larger plants which were previously suppressed by intensive grazing are able to grow. Seeds are dispersed by wind and birds. Deeper rooted plants act as mineral accumulators and other plants act as nitrogen fixers. Soil depths increase and soil erosion and leaching of nutrients decrease. The increasing diversity of plants create habitat for a larger diversity of animals and insects. Herbaceous plants initially predominate until slower growing shrubs and finally trees start to dominate. This process can take decades or even hundreds of years.