Answered Dec 12, 2017
First, you start with a plate (a petri dish), that has agar with nutrients (food) for bacteria.
Then, you inoculate it with some bacteria. These grow and cover the plate,
Then, you add something, such as a little paper disc that has an antibiotic on it.
Then, the antibiotic diffuses into the media around it.
Then, that antibiotic kills the bacteria around the disc. You can see this by looking. There will be a clear-looking circle around the disc.
This is a way to compare different concentrations of antibiotic, or different antibiotic molecules. You can measure with a ruler or calipers.
If you put a higher concentration of the antibiotic, then the cleared area on the plate will be larger.