This refers to the number of simultaneous connections we use when running our speed tests.
"Single-threaded" means that we open a single connection and measure the speeds from that.
"Multi-threaded" means that we're using multiple connections - usually anywhere from 3 to 8 - at the same time, and measure the total speed across them all.
Some connections cannot achieve full speed from a single connection, or have very unstable speeds, and multiple connections allow us to "fill" the available bandwidth more easily. This is particularly true on faster lines. Additionally, many modern applications don't use a single thread when transferring data, so it helps us produce a "real world" result.