This means that a relationship may be good at first but end fast. It's not a good type of relationship, usually very pointless and stupid. In most cases if a person has microwave realtiships they go through several girl/boyfriends in a year. Some analogies are:
-they cook things fast
-They can cook things in 60 seconds when an oven may take 60 minutes
-Things heat up fast and end soon
-There is usually a time limit programmed in
It is pointless for six graders to date because they almost always end up in a microwave relationship.