It is a reference to the English class system.
Lady Bracknell, a stickler for propriety, is suggesting that the Liberals are lower class than herself. The lower classes don't _dine_ in the evening, they merely eat a meal.
_Dinner_ was as much an institution as a simple meal. One dresses for dinner, is summoned by one's butler and is served by one's footmen. She would regard the Liberal Unionists as of a class such as mere _trade_ who don't bother with such things.
So, the Liberal Unionists don't _dine_ \- don't follow the process of _dinner_ \- but they do visit around dinner time.
Lady Bracknell is a snob, there are many instances of her snobbery in the play. Also, the works of Oscar Wilde, an Irishman, contain several jibes at the English class system as he saw it at the time.