A symbol is anything, especially a sign of some sort, that refers to something else in our minds. That means that whenever we see a symbol that has some meaning to us, we are are likely to immediately think of something else connected to it.
In a religious context, the symbol provides us with the meaning of some underlying reality in the world, which can be represented in the form of that symbol.
Generally, we can say that while ritual refers to action, symbol refers to thought or ideas. When we see a symbol, we think of something else. We will see some examples of this shortly.
So ritual vs. symbol is another way of saying action vs. thought (meaning). This is an easy distinction, but it is also basically a theoretical distinction made by scholars, and so the definitions of ritual and symbol could later be changed.