A pioneer species is a species that arrives at the start of a succession sequence. If you’re not familiar with succession, you can find an article about the concept here.
An example of a pioneer species is the Senduduk (Melastoma malabathricum), which breaks up poor soils with its extensive root system and lays down layers of dead leaves which become a carbon rich organic material for the topsoil.
It also blocks out smaller sun loving plants and provides shade for small saplings. By doing so it changes what species can survive in an area and it shapes the direction in which succession can proceed. This plant marks a shift from small herbaceous plants to small shrubs and saplings.
Letting plants take over through natural regeneration is one of the methods to recover soil quality and produce habitat. Being able to identify which pioneer species are there will tell you a lot about the progress of the regeneration.
Not all trees are equal, what, when and where play a very important role in determining whether a tree can support the ecosystem or if it cannot survive in it. Knowing which species act as pioneers is an important aspect of biodiversity enrichment, as it allows us to know when to plant something.