Alliance Help allow other people in your alliance to speed certain tasks. Timers which qualify for Alliance Help include: Buildings, Research and Troop Healing.
The higher your stronghold is the more helps you can receive.
Unconfirmed: From my experience I've noticed that each time you receive "help" your timer is reduced by about 1% of the total time remaining with a minimum of 1 minute of time reduction. At stronghold level 21 you have a maxium of 25 helps. This means your time should be reduced by up to 25% after you've started something that can receive help.