It's a monthly rental fee (
edit: unless they've changed this, the lease period is 24 months). Edit 2: looks like the lease is for as long as you subscribe, unless you purchase the equipment up front. (They lease it because they own it, in the same manner that someone who owns a house leases/rents it to you for a monthly fee.)
Like maratsade said, it's the rental charge for the equipment. This includes the dish and the modem.
The monthly lease fee will be in effect for as long as you have the Gen5 service, though if a newer service is eventually offered and you upgrade to that, that service may have its own lease fee. Only those who have purchased their equipment up front have no lease fee.
You can avoid the rental fee by purchasing the equipment up front. You may want to read this thread about lease v. purchase: https://community.hughesnet.com/t5/Products-and-Plans/Gen5-Equipment-Lease-Vs-Buy-considerations/m-p...