There are a lot of English words that begin with 're'. In some cases re is a suffix that conveys a sense of repetition to the original word and, in other cases the word without re is meaningless or have completely different meanings. I've thought that relax is in the second group but I've recently seen the word 'lax,' and I realized it relates to 'relax.' It means: not strict or careful enough about standards of behavior, work, safety etc. As you can see, the word 'lax' has a negative meaning, which is the opposite of the word 'relax'. @MyPathToEnglish