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Natural selection is Darwin's theory for evolution.
It says that offspring have random mutations in their DNA, which leads to variation. Some of these genetic mutations will cause problems for the offspring, for example they may not be able to produce a certain enzyme. However, sometimes these genetic mutations can help the offspring survive better. For example, if an organism had a genetic mutation that meant it had longer legs and could run faster, it would be able to escape from predators and catch prey more easily. This would mean that it wold have a greater chance of surviving and reproducing to produce offspring that also have long legs.
The same happens with bacteria. When you take antibiotics, there may be some bacteria that has a genetic mutation that means it is resistant to that antibiotic. Therefore this resistant bacterium will survive and reproduce. The bacteria offspring will have the same genetic mutation, so they will all be resistant to the antibiotic. This will mean that antibiotic resistance occurs because the resistant bacteria was naturally selected to survive and reproduce.

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