Antibiotic resistance describes the resistance of bacteria to antibiotics, which are the medicines we use to clear bacterial infections. When someone is sick with an antibiotic resistant infection, it does not respond to antibiotic treatment and can be a large health risk. 

You may also hear the term antimicrobial resistance. Similarly, this is when microbes are resistant to antimicrobials, the medicines we use to treat microbial infections. Microbes refer to bacteria, viruses, fungi, and parasites. 

But when we talk about antibiotic resistance, it is specific to bacteria. Antibiotics treat bacterial infections specifically, and are not used for other types of microbes, such as viruses.

Deaths from antimicrobial resistant infections and other causes in 2050 (Statista)

Antibiotic overuse can lead to antibiotics in our environment, from agriculture, human waste, medical waste, or other sources. More exposure of bacteria to antibiotics leads to more resistance in bacteria.

Antibiotics in the environment creates a pressure for bacteria. The bacteria that are resistant to the antibiotics survive and the non-resistant bacteria die off. The resistant bacteria that survive replicate and pass on their resistance genes.

Antibiotic resistance genes are the genes encoded in bacteria's DNA that make them resistant to antibiotics. Antibiotic resistant genes can also pass between different bacteria species through horizontal gene transfer.

How antibiotic resistance spreads (BBC)