Solar panels are installed in rows, each on a “string.” For example, if you have 20 panels you may have 4 rows of 5 panels. Multiple strings are connected to one string inverter. Each string carries the DC power the solar panels produce to the inverter where it’s converted into usable AC power consumed as electricity.
If you are installing solar panels on commercial-level, then you need several string inverters each receiving DC power from a few strings depending on the size of the installation. On the other hand, if you are installing solar panels on residential-level, then you need to have one string inverter.