This is a dialog from The Graduate. 1967
-Where is Alan St?
-Alan st? It's 6 blocks up and 3 over.
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The explanation actually needs supplementary information to make it complete. Maybe the person in the scene is pointing in a specific direction or gesturing right or left. But without further context it means:
6 blocks up = 6 blocks straight in the direction indicated (probably north).
3 blocks over = (after going the 6 blocks above) 3 blocks to the left or right as indicated.
A block = “a usually rectangular space (as in a city) enclosed by streets and occupied by or intended for buildings” and is also “the distance along one of the sides of such a block”.
“... 6 blocks up and 3 blocks over” = move six blocks ahead and three blocks to the left or right (which of left or right is not defined by the phrase).