The plus/minus stat, sometimes referred to as the plus/minus ratio is a way of measuring how good of a defender a player is.
This stat can be a little confusing. I'll try to clear it up for you. If your team scores a goal while at either even strength or shorthanded, all of your players currently on the ice get 1 point added to their +/- totals (+1). If a opposing team scores a goal while your team is at even strength or on a power play, all of your players currently on the ice get 1 point subtracted from their +/- totals (-1). If a team scores while having an advantage (more players on the ice), neither side's players receive any changes in +/-. All players begin the season with a 0 +/- total.
Players ending the season with their +/- total at a high positive number indicates that they are on the ice more when your team is scoring than being scored upon. Just like a negative total would indicate the player is on the ice more when your team is scored upon than when your team is scoring.