This quote means that if you want to share a truth with someone that they may not like, you can use humor to soften the context. For example, if you are telling someone a hard truth, you can use a joke to make them laugh, so they will be more open to accepting what you are saying. If you just tell someone the hard truth without any humor or levity, they may be upset or angry and may even become hostile. Therefore, using humor can help the difficult truth get across in a more productive and cordial way.