It's a general way to wish others well, usually when parting/saying goodbye. It does not have to pertain to anything specific, but could. Possibly referring to anything coming up in your near future. "Have a good day/night. Have a good trip. Have a good time. Have a good day at school/work. Have a good experience. Have a good life. etc.."
Much like the phrase, "Take it easy". What is "it"?