Short-term, discontinued use of antibiotics provides selective pressure for resistance to the antibiotics. Resistant bacteria survive and then, once the medicine is discontinued, thrive. The resulting post-treatment bacterial population is proportionally more resistant and, thus, more difficult to kill.
Work Step by Step
This highlights the importance of taking all of a course of medicine-- it has evolutionary implications for the population of pathogens attacking you and can make recovery harder or impossible. Evolution plays out everywhere.