A computer program for evaluating chess positions is called an "engine" and you'll find the engine display under the board. You can see the best three options for the player to move as calculated by the engine, an overview of the material balance, the evaluation over time and information about the engine in use.
So what do the numbers -0.23, -0.21, 0.00 mean? The computer assesses the position in roughly units of pawns. Positive numbers favour White, while negative numbers show an advantage for Black. In this example Black is slightly better.
The evaluation over time shows how the evaluation changed during the course of the game. You can see that here Black was slightly better right from the start.
The material balance (how many pawns and pieces the players have) is even in this example.