"They want ice cream."
Translation:De vill ha glass.
Because when you want something, in Swedish you have to specify that you want to HAVE it (e.g. "De vill ha glass" "They want to HAVE ice cream"). However, I think (though I'm not 100% certain) that if what you want involves an action and not an object (such as "I want to go swimming") then "ha" isn't required - e.g. "Jag vill bada" :)
To begin with, if you say in English They want glass, you are not speaking about glass as a container, but as a material. If you want to ask for the container ett glas (notice there's only one s in it!) that would be De vill ha ett glas in Swedish and They want a glass in English.
If on the other hand they want the material glass, maybe for building a house or something, it would be They want glass in English and De vill ha glas in Swedish.
My guess is that it literally means "want to have". In English you can omit the have part when you want an object. "vill" alone kind of means "want" but needs a verb after it. If you want to say "I want to swim" you use "jag vill simma". I hope this is right, if not please someone correct me.