what does "walking down the street" really mean?
Does it means just walking or you have to be actually going down?
if so can you say "walking up the street"?
Good question. Now that I think about, I see it as like a river; upstream means heading toward the source, downstream means heading toward the mouth. So "down the street" might pertain to facing toward the main road (assuming you live on a residential street), or perhaps toward the city center, whereas up the street means either at the street's end or away from the city center, toward the suburbs.
I've never heard "walking up the street." You should say "walking down the street," or "walking along the street."
Walking down the street can be synonymous with walking along the street—you don't have to actually be descending. Walking up the street is more rare and is more likely to mean that you are actually going upwards, but it can also mean that you are walking in the "up" direction of a map (usually north) or that you are walking towards the center of the town.
Both up and down are often used figuratively in such contexts, and which direction is used for which purpose tends to depend on the region being discussed. In some cities, people go "down" to the center of town; in others, they go "up"—and this may or may not correspond to actual elevations.
I think there's also direction relative to the speaker. It may not apply every time, so context is important.
"John is walking down the street" could imply walking away from us.
"John is walking up the street" could imply walking towards us.