Simply put, a string inverter is a device for converting DC to AC power and which is designed for high voltage DC inputs.

Single Axis Solar Tracker Back

Using a string inverter, the solar panel array, still typically rated at 12V, 24V or 48V each panel (although higher voltage panels are now coming out) is wired in series, rather than in parallel. It's that simple: The panels are arrayed in a "string" to produce the same amount of total power but at higher voltages (typically 200-1000V), thus lower current, allowing much smaller wiring and much smaller and lighter weight inverter construction. The IR losses encountered in wiring are also present in the inverter equipment's electronics, thus higher voltage DC input circuitry can be built more economically and with lower internal losses, improving efficiency in the electronics as well as in the cabling between the solar array and the inverter.