In a nutshell, a Draw Call is simply a set of CPU commands to display a mesh, which tells the GPU what and how to render (draw). It contains information about the States (e.g GPU/Render State).
For Artists the main concern is probably the number of materials used on a mesh, as every single material will issue a new draw call.
Since such a draw call is not free, too many of them can put a huge burden on the CPU and therefore cause serious performance issues. Typically it's the best advice to keep the total amount draw calls in the 2000-3000 range on current hardware.