Other forms: going the extra mile; went the extra mile; gone the extra mile; goes the extra mile
To go the extra mile is to go above and beyond what would normally be expected. If a store doesn't have the item you're looking for, a salesperson might go the extra mile by calling all over town to find a store that has it.
A salesperson isn't obligated to call around town to help you find a product at another store. Doing so is just very helpful and very nice. The phrase go the extra mile comes from a story in the Bible. Soldiers in the occupying Roman army could force any passerby to carry their heavy equipment for one mile, but Jesus told his followers to go two miles — an extra mile — of their own free will, in order to restore their dignity while blessing the soldiers. Now, going the extra mile means doing more than your duty, in any context.