noop stands for "no operation". It means you are passing some parameters to the noop command, where the parameters on their own aren't very important, rather the consequences of evaluating these parameters. So basically you are telling mirc: I want these parameters to evaluate, but I don't want to do anything with the results of the identifiers (don't do an operation with the results)
echo -a $regml(1)
noop $findfile($mircdir,*,0,echo -a $1-)
echo -a $com(object).result
In all cases we are not very interested in the results of the identifiers, but we are interested in the consequences of letting them evaluate. For the regex, we needed to issue it in order to fill $regml, for the $findfile we wanted to let that echo execute on each file without doing anything with the total number of files (which is what $findfile returns in that example). The example with $com will retrieve a property from a certain object, which is stored in $com(<obj>).result, but in order to get this result we first gotta make that call to $com evaluate.