What does the idiom one in mean?

The idiom "one in" is typically used in conjunction with a number to express a very low probability or chance.

For example:

* "One in a million" means something extremely rare or unlikely to happen.

* "One in ten" suggests a 10% chance of something occurring.

It essentially means that for every "x" number of events, something will happen only once.

Here are some other examples of how it's used:

* "The odds of winning the lottery are one in a billion."

* "There's a one in five chance of rain today."

* "The doctor said there's a one in ten chance of complications."

The exact meaning of "one in" can vary depending on the context, but it always signifies a low probability.