Pioneer are the first organisms (vegetative) to grow or establish themselves in an area experiencing primary succession which allows for a foundation of other subsequent growth to occur.
Primary succession is essentially starting from nothing. Picture rocky terrain such as a new island or a lava covered landscape.
Pioneer species include things like algae (a protista) mosses and lichen. These will over time establish sediment by loosening rock and creating organic biomass for the soil as they die and decompose. With the establishment of soil more complex plant species – glasses weeds and eventually shrubs can grow as well.
It is a misconception that secondary succession – areas starting over from a disturbance like a fire or flood – have pioneer species. Secondary succession never starts at the beginning with no soil layer to establish plant life therefore there is technically no need for the pioneer species mentioned above. Secondary succession can skip ahead and start with grasses etc.