I know that the nodes are the places where the leaves are, and that it`s also the place where buds form, but what the hell does `short internodal spacing`mean, and how can one encourage such a thing to occur?
the stem length between leaf/branch nodes is "internodal distance" or the distance between nodes. the main way to encourage short internode lengths is to get plenty o f light to your plants. this usually means moving lights closer to the plants when growing indoors. recommended distance for fluorescents is usually within 2" of the top of the plant. i'm not sure what it is for HIDs since i've never used them. some of the internodal distance is determined genetically i think, and there may be other factors affecting "stretch" (another term used to refer to elongating internodal distances.
Just because your lights are not adjustable doesn't mean you're screwed. MH bulbs will give much closer 'internodal distance' than HPS when used in veg. The bluer the light spectrum (T5 or MH) the closer the node spacing will be. If you veg a control plant under either MH or T5 fluorescents and veg another for the same amount of time under a HPS you will see the difference. The plant under the HPS may be taller than the MH/T5 one, but it will be stretched, and the nodes will be farther apart.