"explain how both the principles of superposition and original horizontality are important to determining the history of the rock layers."

The principle of superposition is that in a horizontally bedded sequence, the lower strata are the oldest and the top strata are the youngest. The sedimentary rocks were originally laid down horizontally so preserve features that are a consequence of this horizontality so if the strata have been uplifted and perhaps tilted say to vertical, using such clues as original graded bedding ie more coarse grained seds at the bottom of beds and progressively finer grained seds above them within a stratum, this is a clue as to what is up in a vertical or overturned sequence. Also, the fossil content will tell which strata are younger and which older.