Szia wanipa!
Ez lámpa. -> simply tells you the name of the object (e.g. if you don't know the name of that thing, you are given the solution).
Ez a lámpa -> in this case - for whatever reason - "lámpa" (or that "lámpa") has already been mentioned earlier, so you are not only given the name of the object but also an indication that you and the answer giver know "what" lámpa you are talking about. Alternatively, if it is understood that you are talking about objects in the room you are in, the object in question can be considered as definite for that reason.
Or you can consider that "Ez lámpa"="This is a lamp." meanwhile "Ez a lámpa"= This is the lamp" (we were talking about earlier, or is definite because we are talking about the objects around us, not about lamps/names objects in Hungarian, in general.)