'Ground' is the past and past participle of the verb 'grind'. You have to grind coffee beans before you can use them. 'Ground coffee' is coffee beans that have been ground down, ready to be put into a machine (espresso machine, cafetiere, percolator etc) and brewed to make a cup of 'real' coffee. 'Real coffee', colloquially speaking, is the opposite of 'instant coffee'. 'Real coffee' is made from 'ground coffee', as opposed to 'instant coffee', which is made by adding water to a spoonful of Nescafe.