A meltdown is a childs reaction to something that is usually beyond a child's control and the meltdown will not usually stop when the child gets what he or she wants.
Tantrums are done because the child wants something such as a toy, candy etc and if you give into the child when they are having the tantrum they will usually stop the tantrum because they got what they were having the tantrum for.
The tantrum is basically a way for the child to try and force the issue of getting something they want.
But you should never give into a child's tantrum even though it may seem like the best thing to do.
When you give into the child to stop their tantrum the child will stop the tantrum but the child will also learn that if they just have a tantrum that they will get what they want.
So you will be creating more tantrums but giving into the child.
Just be calm and tell the child no and remember you're not the only parent who's had a child have a tantrum in a store and you and I had tantrums in stores and in public places as well even though we might not remember it.
Tantrums are embarrassing but you're not alone and every parent deals with tantrums from their children.