Let's start with some terms:
"Solidus" refers to the temperature that an alloy melts at. "Liquidus" is the temperature that the alloy turns completely liquid. Solids turn directly into liquids when they are heated, right? Not exactly.
When formulating alloys, there are usually one or more points in the constituent ratio where the metal will be at a "Eutectic" ratio. This ratio is usually the lowest melting point for the different combinations of those elements. For example, if you mix 63%Sn with 37%Pb, it will have a single temperature (183ºC) for both its solidus and liquidus. If you mix the alloy with a different amount of Sn or Pb, the solidus may remain while the liquidus increases. The range between these temperatures is often called the "pasty range". During heating between the solidus and liquidus, most metals act very much like a liquid. The resulting mixture of liquid and solid material is able to wet to surfaces and form intermetallics, although it is recommended that soldering be done above the liquidus point.
More information on measuring temperatures from Differential Scanning Calorimetry, check out our Application Note on Determining Solidus and Liquidus points.