Skipping over fractional game values for the time being, let us talk whole numbers.
We will start off considering games blocking games where the first person without a move loses. By convention, the Left player (which I'll name Lucy) will win if the game's value is positive, >0, or, in other words, has move moves than the opposing side. And the right player (Robert) will win if the game's value is less than 0, more moves than Lucy.
If the game has a value of 0 (think of a game board without pieces), then the first person to move is the loser (no piece to move).
Follow along with Berlekamp, Conway, and Guy in "Winning Ways".