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 200-800V), 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.
The popularity of this new approach in PV (photovoltaic solar power) applications has not fallen on deaf ears in other arenas, as well. Small-scale wind generators, for instance, have followed a similar evolution. Starting with 12V and 24V automotive-type generators or other low voltage DC units and batteries, the more current crop of home wind generator systems are now using higher-voltage principals and, for grid-tied systems, are skipping the batteries, too. The term "string inverter" is somewhat nonsensical in this context because the small-scale wind generators are not typically done in arrays like solar panels and there is typically only a single generator per installation (for small systems). Now high-DC-voltage "grid-tie string inverters" are all the rage for use in wind systems, too.
So, string or no string, the bottom line is that the budding homebrew power industry is getting hip to high voltage as a means of improving system and inverter efficiency and lowering the costs of getting the power from one place (the roof or tower, etc.) to another (the location of the inverter). String inverters should more accurately be called simply "high input voltage inverters", probably, since the "string" moniker really applies only to multi-panel PV solar systems and these same units are very popular for wind and hydo-generation systems where there is no "string" of series-connected sources, per se.