These are Islamic legal rulings.
Mandûb and mustahabb are terms that refer to the same ruling. This ruling means “preferable” or “recommended”. It can be defined as that which we are requested to do without being obliged. It is something we are rewarded for doing, but we will not sinful for neglecting it.
An example would be performing the Duhâ prayer in the morning or fasting on Mondays and Thursdays.
Makrûh means “disliked”. It can be defined as that which we are requested to abstain from without being obliged to do so. It is something we are rewarded for avoiding, but we will not be sinful for doing it.
An example of this would be speaking unnecessarily while going to the bathroom.
And Allah knows best.