Resistant bacteria do not respond to the antibiotics and continue to cause infection. Each time you take an antibiotic unnecessarily or improperly, you increase your chance of developing drug-resistant bacteria. So it is really important to take antibiotics only when necessary. Because of these resistant bacteria, some diseases that used to be easy to treat are now becoming nearly impossible to treat. Remember that antibiotics don't work against colds and flu, and that unnecessary antibiotics can be harmful.
Antibiotic resistance is particularly dangerous for children, but it can occur in adults as well.
One final note is that taking antibiotics appropriately and getting immunized will help prevent having to take more dangerous and more costly medications. If we use antibiotics appropriately we can avoid developing drug resistance. We just need to take our medicine exactly as it is prescribed and not expect to take antibiotics every time we're sick.