'Frenzy' was introduced in the Time Spiral block, it is only on one card which is 'Frenzy Sliver.' It is a Future Sight card suggesting that we will see 'Frenzy' again in the future.

702.65. Frenzy

• 702.65a. Frenzy is a triggered ability. "Frenzy N" means "Whenever this creature attacks and isn't blocked, it gets +N/+0 until end of turn."
• 702.65b. If a creature has multiple instances of frenzy, each triggers separately.

Translation:

• 702.65a. - It's a basic combat related ability, attacking or blocking gives it +N/+0
• 702.65b. - Frenzy stacks, so multiple Frenzy's make multiple bonuses.