で is a particle that has several different uses in Japanese. At its core, it is always used to highlight something that is 'required' to perform some sort of action. One of these uses relates to places, and translates roughly as 'at', or 'in' in English.

• 私（わたし）はここでピザを食（た）べる。
  I eat pizza here. (At this place)
• 私（わたし）は部屋（へや）で寝（ね）る。
  I sleep in my room.

In these sentences, the place is considered essential for the action to be performed, or the result of the action to be achieved. This is unlike the particle に, which marks the location itself as being the destination or target of an action. Basically, で means (A) 'place' allows (B) 'action' to occur. While に means (A) 'place' is the target for (B) 'action' occurring.

• 彼（かれ）は公園（こうえん）にいます。
  He is at the park. (He is at the park, but the park isn't 'allowing' him to be there)
• 彼（かれ）は公園（こうえん）でバーベキューをする。
  He BBQs at the park. (Because there are barbecues at the park, it is allowing him to cook)

In the first example, we can see that に marks the place that the person exists during an action, but the action itself is not dependent on the place. In the second example, the action is considered dependent on being at that place, as there are possibly no barbeques in other areas.