Lúcioball is a seasonal event, it is only available for three weeks in late summer (usually in august). The game mode is found in the arcade menu. Two teams of three try to score as many goals as they can until the time runs out. Everybody plays Lucio who has slightly different abilities. He can punch the ball (RT), shoot the ball (LT), skate on walls and jump on jump pads which make him move faster around the arena. Lucio also has his speed boost ability (RT) and his ultimate makes him extremely fast for 10 seconds.
A Save is when someone stands on the yellow territory in front of his own goalpost and punches or shoots an incoming ball away which was sent by someone from the enemy team. You can make a save by jumping as well, if you don't leave the yellow mark. A successfull save is indicated by both a blue SAVE word popping up on the screen and by the announcer saying the same. You can also check it in the statistic (hold the left small button). While this may come naturally - when the ball lingers around the goalpost too long and the attackers just try to force it in with repeated shots, the easiest way is - as a goalkeeper - to play 1v1 with the enemy goalkeeper. This means you look up very high and shoot the ball with LT towards the enemy goalkeeper. The ball goes so high there is little chance anyone can touch it but the goalkeeper who tends to shoot it back at you with LT as well. You can then give him back and this could theoretically go on for 5+ times easily. Every time you shoot it back it counts as a save if you don't move from your goalpost.