五十歩百歩 literally means “fifty steps, a hundred steps”, but we use it more in the vein of, “basically the same thing.”
This phrase comes from an old Chinese story. There were two soldiers who retreated from the battle - one of them moved back by taking 50 steps, the other who moved back 100 steps. Then the first soldier says to the second soldier, “You moved back 100 steps? What a coward!” - the joke being that either way, they both retreated!
It is more common to use 五十歩百歩 when describing silly and low level competitionss. We don’t really use 五十歩百歩 for the serious, high level competition, such as the top athletes in the Olympics.
五十歩百歩 is an idiom, but we often treat it like a noun (or sometimes な-adjective) when using it in a sentence. So for example, we can say either 五十歩百歩の競争 or 五十歩百歩な 競争 to mean “basically the same kind of the competition.”
Mary’s Japanese and Kayoko’s English are basically at the same level (i.e. not very good).
Although you drink cola instead of alcohol, it is basically the same. Both are bad for your health.