Long coupons, as the name suggests are coupons with large time of maturity, normally two years.
Sometimes long coupon is also referred as the the coupon where the first period is longer than other standard periods.
A bond with 5 years till maturity is a long coupon/bond or a 30 year bond . Usually if the time period which is in years, it is referred to as Long Coupons.
They are also sometimes referred to as Long Bonds.