What does 1.4.x/1.5.x mean?

Many people seem to be confused what "x" means.
"X" is implied as a wildcard version number.

When you see someone say their mod works with "1.5.x" they mean it works with ANY version of OpenCart 1.5
1.5.0, 1.5.1, 1.5.1.2, etc

1.4.x of course means any version of OpenCart 1.4

It can be fine tuned as well

1.5.1.x means it works with
1.5.1.1 through 1.5.1.9
but not necessarily with 1.5.2 or 1.5.0 etc