Speaking a language is like a game of golf.
Hit the ball too far left, and it ends up in the lake.
Put your tongue too far forward, and you end up saying “beau cul” instead of “beaucoup” in French (“beautiful ass” instead of “a lot”).
“Beau cul” in IPA is [boky], and “beaucoup” is [boku]. The difference between [y] and [u] in French is the position of the tongue. In the back of the mouth for [u], and in the front for [y]. You can hear the difference here on Forvo.
In both cases, a few millimeters make a great difference.