Relative humidity is a sort of left-handed way of looking at dewpoint. The dewpoint number is what meteorologists use in their calculations, but relative humidity is what they give to Joe Sixpack.
The simplest explanation of dewpoint is the temperature at which relative humidity reaches 100%. What changes it is the amount of water vapor in the air. The less water vapor, the lower the temp has to be to achieve saturation (100% RH) and therefore condensation.
Just the RH number tells you very little. You can have an RH number of 50% or any other percent with next to no water vapor or a bunch of it. That's why dewpoint is more useful to the experts.
Density altitude relates to atmospheric pressure. Air becomes less dense as altitude increases, as we all know. But because the pressure at the surface changes, what you read on your plane's altimeter may not report your real height above the ground. So density altitude is a correction for the present barometric pressure. Pilots set their altimeters to a reported barometric reading so they know their real altitude.
