what does docker version name means?

The number before the colon, 3: in this case, is an epoch number, used to handle situations where the version number needs to be decreased. This could be the result of a beta or RC build getting out with a higher version number that wouldn't allow a normal upgrade to the GA release. I believe you need this in any commands selecting version numbers, but you can otherwise ignore this when trying to understand which version you have installed.

The important part of that version number is 18.09.5 which tells you:

• 18.05: the major release number from docker's rolling releases, they are roughly based on the date of the first patch of the release, so 18.05 refers to May 2018
• 5: the patch number, where fixes are released without new functionality

Note, directly from docker, you can run:

docker version

To see both the client and server version numbers.