Antibiotice resistance is one of the biggest challenge that modern medicine is facing. To answer the question above let us first explore our understanding of why we use antibiotics and how it works.
Antibiotics are substances used to help fight bacterial infections. 'Anti' means against and 'biotics' means living organisms. These substances function in various ways. For example, a group of antibiotics called macrolides inhibit bacterial protein biosynthesis; a biological process which is essential for the survival of the bacteria. Where as another group called the beta lactams act by damaging the cell wall.
When we use antibiotics irresponsibly we can yield resistive organisms. How does this happen then? Imagine a bunch of balloons (the disease causing bacteria) at a party. Then imagine a set of needles (the antibiotics) that some children have gathered. Initially the needle is able to burst all the balloons easily. At a different party the ballon vender happens to have one extra thick balloon amongst the bunch of balloons. He notices that the needles are not able to burst that partcular type of balloon. Then he brings more and more of the new type of balloon THAT IS RESISTANT to needle prick and keeps the decorations alive through out the party.
So one bacteria within the group that happens to survive due to some difference in its properties divide and reproduce to form a new set of bacteria that are resisntant to that particular antibiotics. We can reduce the progression of this happening by mixxing up ntibiotics and makig sure no bacteria survive to reproduce and form a resistant colony.