The 18 holes on a course are generally ranked according to how hard they are using the stroke index.
Usually this means that the hardest hole on a course will be stroke index one and the easiest, stroke index 18.
But, as with many things in golf, it is more complicated than that.
Put simply, the stroke index works in parallel to the handicap.
So if you have a handicap of 14, you should subtract one shot from your gross score at the holes which have been designated as stroke index one to 14.
If you have a handicap of more than 18, there will be some holes at which you can take more than one shot.
For instance with a handicap of 24, you should take two shots at holes with stroke index one to six (18 + 6 = 24).
Although holes are rated by difficulty, the even spread of the stroke index is the most important factor.
If you look at a card the odd numbered stroke index holes will be allocated to the harder of the two sets of nine holes on the course.
There are other factors for deciding the stroke index. You can see these at the English Golf Union's website.