A medication's half-life is the time it takes for the plasma concentration of a drug to reach half of its original concentration. More simply put, the half-life of a drug is how long it takes for half of it to be eliminated from the bloodstream.
One impact of half-life is found in the SSRI antidepressants. People taking SSRIs with short half-lives are much more likely to experience SSRI Discontinuation Syndrome. People taking an SSRI with a long half-life such as Prozac need to wait far longer between stopping Prozac and starting to take an MAOI antidepressant (see MAOI Antidepressants).