Roof Pitch, commonly known as "slope", is defined by how many inches the roof rises across 12 horizontal inches. Also known as Rise over Run or Rise/Run. For an example a 7/12 pitch roof means that the roof rises 7" for every 12" it runs.

To measure your roof pitch, mark a level at 12" from the top of the roof. Hold it perfectly level and then with a tape measure, measure the rise of the roof.
This can also be done from in the attic using the rafters as your slope guide to measure from.

The examples below illustrate Rise/Run roof pitch.  Notice the smaller the first number (rise) the shallower the roof pitch (slope).