The agrobacteria insert this new DNA into a huge number of cells. Some of these cells (the egg cells, specifically) are the cells that form seeds which will grow into new plants. Therefore, it is this next generation of plants that may contain the insertion! Not all of the offspring will have this insertion, however. In order to only continue studying the plants with the insertion, an extra piece of DNA is included in the plasmid used in the second step. This extra DNA encodes for resistance to a specific antibiotic. Basically, a huge number of seeds will be sown out on either soil or agarose that contains a specific antibiotic and only the seeds that have the DNA encoding resistance to this particular antibiotic (and therefore the seeds that also contain the DNA that we are interested in) will grow!