What does #(nop) mean in docker history?

NOP stands for "no operation".

Docker "runs a shell" for every layer. All the docker commands (or layers) in the Dockerfile except the RUN command show up in the history as empty or commented out shell commands. The # sign marks the start of the comment and anything after that will be skipped by the /bin/sh. Similarly if you typing in the terminal:

user@machine $ echo hello world
hello world
user@machine $ #But none of these lines starting with # do anything
user@machine $ #echo hello world

The non-RUN commands will not need to be interpreted by the shell, but instead are processed by docker internally.

The history (including the non-RUN commands) can be used by the layer cache to skip processing in case the same command has been run previously.

FROM ruby:2.6-alpine
ENTRYPOINT ["sleep", "infinity"]
ENV A 1

$ docker build -t my-useless-image .
$ docher history --no-trunc my-useless-image
... CREATED BY                                         ...
... /bin/sh -c #(nop)  ENV A=1                         ...
... /bin/sh -c echo test                               ...
... /bin/sh -c #(nop)  ENTRYPOINT ["sleep" "infinity"] ...
...

For RUN commands, it displays the shell command it executed, e.g.

/bin/sh -c echo test

For non-RUN, /bin/sh -c #(nop) followed by the Dockefile command that was performed in that layer. /bin/sh -c #(nop) doesn't really mean anything useful, and can be ignored. They made it look like a shell command, but it would fail if executed.