MIC = Minimum Inhibitory Concentration = the lowest amount of antibiotic required to kill the bacteria.
Antibiotics either kill the bacteria or slow it down enough that your own immune system can get rid of it.
Antibiotics are designed to maintain a certain amount of medicine in your body over a certain number of days to ensure the infection is completely gone.
Do not take antibiotics that you have left over from previous treatments, because you most likely do not have enough medicine for a full course of treatment.
Think about a spot of dirt on the floor:
Dirt + a few drops of water = mud
Dirt + a whole pitcher of water = a watery mess
Dirt + a wet rag = clean floor
Relate it to an infection:
Infection + too little antibiotic = resistance
Infection + too much antibiotic = side effects or toxicity
Infection + the right dose of antibiotic = you get better
The gap between the lowest effective dose and the highest, non-toxic dose is called the Therapeutic Index.
That is why you should always take antibiotics exactly as prescribed and until they are all gone.