One-size-fits-all (/wɒn saɪz fɪts ɔ:l/) is a phrase which literally describes an item of clothing designed to be worn by all adult humans of sizes within a normal range. Additionally, it has an idiomatic meaning, that of a service or product other than clothing that is provided to everyone in an attempt to cover their needs, regardless of their individual needs.
A one-size-fits-all approach should theoretically keep everyone happy. However, in real life people differ significantly from norms. So, what you get is a "one-size-fits-most" approach which will keep most people happy most of the time, but the implication is that it may not keep everyone happy all of the time.
In a TEFL setting, many courses may attempt to be one-size-fits-all, but turn out to be one-size-fits-most as they are not tailored to each individual's needs, but to a hypothetical typical or average student. If a student is atypical, such as having high or low aptitude, this may not be appropriate for them.