Single pane windows, usually found in older homes, are made with one layer of glass. If you look closely from the side you can usually tell if there is a single pane of glass or two (or three) panes with space between them, often this space is filled with insulating argon gas. Occasionally there will be an additional removable pane of glass that is commonly known as a storm window.
Single pane windows do not offer very good protection from hot summer days or cold winter nights. This is because heat is able to easily transfer through the un-insulated glass. Double and triple pane windows have an insulating layer of air or argon gas between the panes. This slows down the heat transfer through the glass and helps to keep your home cool in the summer and warm in the winter.
Whether you should replace your single-pane windows comes down to the specifics of your home, including the amount of outside noise, the amount of money your regularly pay for heating, the age of your home, and a few other factors. If you want lower and more predictable heating and cooling bills, and reduced outside noise, then upgrading your windows may be a good option!