Ablaut, by my reading, is the phenomenon of vowel gradation (phonetic variation) within related words of the same language, derived from parallel variations in the parent language.
Some English ablaut variations:
- ‘strong verbs’ : sing, sang and sung / ring, rang and rung
- nouns : man, men / goose, geese
This same variation exists within Latin:
- tegō, I cover; toga, robe
- pendō, I weigh; pondus, weight
- fidēs, faith; fīdus, faithful, foedus, treaty
- regō, I rule; rēx, king
- dūcō, I lead; dux, leader
Ablaut will often demonstrate grammatical demarcations between nouns and related verbs, but also between various tense-stems of the same verb:
- cadō, I fall; cecidī, I fell
The Essential AG: 17
I knew nothing whatsoever about ablaut before designing this post, so if any visiting linguists would like to expand in the comments below, I encourage them to do so.
Latin for Addicts 
Really helpful post, thanks!
Glad to hear it, Ravi!