An encyclopaedia of chess openings is like a big book that teaches you all sorts of ways to start playing chess. When you start a game of chess, you need to know how to move your pieces in the right way to try and win the game. The book teaches you lots of different ways to move your pieces at the start of the game, which is called the "opening".
Imagine you are playing with your toy cars. Just like you might start the game by putting your cars in different positions, in chess you also start by moving your pieces around the board. The encyclopaedia of chess openings tells you all the different ways you can move your pieces at the start. For example, one way you can move your pieces is to move your pawn forward two spaces. Another way is to move your knight out to the side.
Some people really like to study the encyclopaedia of chess openings so they can get better at chess. It's like practicing and learning new tricks to try and win. Just like you might practice with your toy cars to get better at winning your games, people practice with the encyclopaedia of chess openings to get better at chess.