Game Mods are mostly used for achieving more score during ranked play, but with some you can make a certain track easier or you can just watch the track complete itself.
There's 3 different kinds of mods - Difficulty Reduction mods, Difficulty Increase mods and Special mods.
The easy mod makes the track.. well... easier. The circles become bigger, there's less accuracy required and you lose less HP when you make a mistake. You only gain x0.50 score while using this.
The No Fail mod speaks for itself aswell. Once your performance meter drops you can still keep going. Good for practicing certain tracks. You only gain x0.50 score during this aswell.
The Half Time mod makes the track go twice slower than it should. It only grants you x0.30 score.
Hard Rock mod makes hitting notes more difficult, they're smaller and you need pinpoint accuracy for 300 points on the approach circles. x1.06 bonus multiplier.
Sudden Death mod makes you fail after one missed note. It grants you no extra points, so it's just a faster way to retry if you're going for perfect score.
Using the Double Time mod the track will go twice as fast. It grants the most extra points - x1.12 score multiplier.
The Hidden mod removes the approach circles and makes the notes fade. Use this only if you know the track really well. Grants x1.06 extra score.
The Special mods are all unranked and grant no points. There's 4 of them : Relax, Auto Pilot, Spunout and Auto. They all make the track way easier and there's no need of an explanation for any of them.
You can use multiple Game Mods to further increase your Score Multiplier and go for High Scores!