I read a blog post about dehydration. But I'm still not clear - what is it?

Easy/hard answer! "dehydration" is a loss of water from the body, but is incorrectly used for the more appropriate term of "volume depletion" which is loss of salt and water, and commonly occurs in hot environments, excessive sweat loss, diuretic use, or lack of access to water. It can often be treated with fluids by mouth or IV sodium chloride under the direction oh health providers. Avoiding causes is key.
Loss of body water. Technically, dehydration means losing water without losing the salt like when you sweat much in hot weather and not make up for it with drinking water due to nausea or vomiting. Now, if you lose also the salt e.g. When your water pill works well (losing salt & water) and you have nausea/vomiting docs call this volume depletion(volume=salt+water). For lay persons you may call that dehydration too.
Inadequate fluid. Dehydration is a condition of relative loss of fluid compared to intake in a relatively short period of time. Total body fluid is low and can affect both kidney and cardiac performance. The salts (electrolytes) in the blood can be disordered causing further problems in normal organ function. If not corrected shock and total kidney faiure can occur.