Random walks in Feller's 'An introduction to Probability Theory and Its Applications'

Question

In the book 'An introduction to Probability Theory and Its Applications' (3_rd ed.) by William Feller, the 3_rd chapter on random walks starts by considering a situation where we have

n = p + q symbols (x₁,...,x_n) standing for either +1 or -1.

Then the partial sum s_k=x₁+...+x_k and s_k-s_k-1=x_k=+/-1

After that, he writes the following (page 68):

"The arrangement (x₁,...,x_n) will be represented by a polygonal line whose kth side has slope x_k and whose vertex has ordinate S_k. Such lines will be called paths."

Up until this point, I have found this book to be extremely helpful and illuminating, but the first sentence really confused me.

If anyone could explain the terminology, the definitions or just illustrate the problem in some way, I would be grateful.

Thank you in advance.