SMARTnet is a hardware and software maintenance contract for Cisco equipment, supplied directly from Cisco. The contract may be sold by authorised Cisco Partners, and is normally an annual contract; longer term contracts are available, please contact us for more details.
Quite simply: It's a maintenance contract for Cisco equipment.
If your hardware breaks, Cisco replace it.
If you need support for any of your equipment, you get the right to call the Cisco Technical Assistance Centre.
You also get the right to download and install software updates for your Cisco equipment.