The phrasal verb "to go up" has a number of different meanings; I knew which one it had within the context of the sentence but to my surprise, it was difficult to find an online dictionary which listed that precise definition. Thereby, explaining why the OP couldn't "get the exact meaning". Here the expression to go up 5 means to reach a destination (usually by vehicle) that is higher up.
He went up to Mary's at about 8 o'clock.
has a certain ambiguity that without knowing what happened earlier or later in the story could possibly mean three different things.
The first one, the man was already at Mary's house before 8 o'clock. Perhaps he was her husband, boyfriend, brother, or father we don't know. Nevertheless, at about 8 o'clock he went upstairs to Mary's room. Maybe she was unwell and he wanted to check if everything was all right. Again without more information, we cannot know.
The second interpretation, and the most likely, is the following; at around 8 o'clock he drove/travelled up (northward) to Mary's house. If she lived southward or at the bottom of a hill, the sentence would start with: *He went down to Mary's (house)
Finally, from a third and British perspective it could be that Mary lived in a big important city, regardless of its geographical position so, paraphrasing, we end up with "He went up to [London] where Mary lived."