You'll have found by now that algebra forms a major part of KS3 Maths. In algebra letters are used in place of numbers. The letter n (usually in italics) is often used to indicate the position of a term in a sequence. We call this the 'nth' term.
Finding the rule for a number pattern is usually quite easy but can sometimes be a bit harder. One famous pattern is the Fibonacci sequence. This describes a spiral pattern and is very common in nature. The rule for the Fibonacci sequence is to add the previous two numbers to find the next. The nth term for this would be... advanced mathematics. Don't worry about that just yet!
Here's an easier example. If a sequence begins with 5 and goes up in twos thereafter, the nth term of that sequence would be 2n + 3 (2 x 1 + 3 = 5, 2 x 2 + 3 = 7, 2 x 3 + 3 = 9 etc.). This can be a little difficult to grasp but work through this quiz (and read the helpful comments!) and you will soon get the idea.