What makes an indoor cycling trainer "smart"? It is the ability to transmit a data stream to your device enabling you to use the data for your app or program. Smart trainers transmit a Bluetooth or Ant+ signal or both. The Bluetooth signal will connect with a tablet, phone or PC that has Bluetooth built in, whereas the Ant+ signal is for other devices (mainly PC's) and will require the additional purchase of an Ant+ dongle or receiver. This connects into a USB port and receives the signal.
It is important to note that purchasing a smart trainer is no guarantee it will work with your device. Most buyers want a "plug and play" experience but when you are using disparate pieces of hardware and software, and making them work together, it is not always seamless. You must have the app or program you would like to use installed and updated, and older devices may not simply not be compatible with the software. If you are using software that comes with the trainer setup can be easier, but using apps such as Trainer Road, Zwift, or Straava may require additional steps to get you where you want to go.
Some trainers are "smart upgradable" meaning you can purchase a wireless transmitter that transmits speed and cadence and extrapolates power. Less expensive smart trainers, or upgradable trainers do not measure power but estimate it by using cadence and gearing. If you are looking for an accurate indoor power meter these types of trainers are less accurate than one using actual strain gauges to accurately measure power. More expensive smart trainers have electronic braking that can simulate grades, courses, even rough road conditions. These trainers offer the most interactive experience but come at a premium. Smart trainers are wheel mounted or direct drive. Wheel mounted trainers attach to your rear skewer with a resistance roller contacting your tire. These are easy to setup and remove but tend to eat up your tire causing a potentially dangerous flat spot if used outdoors. Direct drive trainers require removal of your rear wheel and mounting your bike on a cassette that is integrated into the trainer. These are again more expensive but have more features and are better simulations of road conditions.
A smart bicycle trainer offers you the ability to train on your own bike in a fully interactive environment. If you are a cyclists or competitive cyclists the first rule of training is specificity; and an indoor spin bike on an ill fitted bike is not nearly the same as riding your own. For this reason smart trainers have seen incredible growth in popularity, especially now that open source apps can be integrated.