There are a number of mechanisms through which a bacterial cell might become resistant to a specific antibody.
Some bacteria might posses a cell membrane which wouldn't allow the transfer of antibiotics into the cell of the bacteria. Some bacterial cells might not have the target sites on which the antibiotics attack. A bacterial cell that would be non-resistant wouldn't block the entry of the drug or it might not lack the site where the drug has to function.
Some resistant bacteria might produce enzymes that would inactivate the activity of the antibiotics. This enzyme machinery would lack in a non-resistant strain. The resistant bacteria might have undergone mutations over a period of time to evolve a machinery for antibiotic resistance.
A population is a group of the same species within a given area. If you have 50 flamingoes, that's a population. If you have 50 flamingoes and one duck, that's not a population, it's called a community.
