What is an Antibiotic? By the book, it's a "substance produced by one microorganism that selectively inhibits the growth of another." But what does this mean?

Microorganisms manufacture "weapons" to use against each other. The medicines that we use, to fight bacterial and fungal infections, take advantage of these weapons....in essence, to beat them at their own game. By weakening the defense of a bacterium, an antibiotic makes it easier for the body's immune system to clean up the infection.