A MRE consists of the following items:
a minimal dataset, necessary to demonstrate the problem
the minimal runnable code necessary to reproduce the issue, which can be run on the given dataset
all necessary information on the used librarys, the R version, and the OS it is run on, perhaps a sessionInfo()
in the case of random processes, a seed (set by set.seed()) to enable others to replicate exactly the same results as you do
For examples of good MREs, see section "Examples" at the bottom of help pages on the function you are using. Simply type e.g. help(mean), or short ?mean into your R console.