E. coli are a species of bacteria. Like all bacteria, they have a round piece of DNA that contains most of their genetic information, but also plasmids, that contain some more. Plasmid can be passed on from bacterium to bacterium and they contain information for fighting antibiotics. Tetracycline is such an antibiotic. The process of putting a plasmid inside a bacterium is called transformation. So what the above sentence really says, is that a bacterium just got a plasmid that is probably holding the genetic information to produce proteins that fight antibiotics like tetracycline. While regular strains of E. coli might not be able to deal with tetracycline, transformed strains will probably be able to survive it. Hence, the culture will have live bacteria and the bacteria that have a plasmid against tetracycline will be favored and survive.