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There's a complete explanation at http://www.bartleby.com/61/8.html. Here's the basic part of it: ===== A characteristic feature of Indo-European was the system of vocalic alternations termed apophony or ablaut. This was a set of internal vowel changes expressing different morphological functions. A clear reflex of this feature is preserved in the English strong verbs, where, for example, the vocalic alternations between write and wrote, give and gave, express the present and past tenses. Ablaut in Indo-European affected the vowels e and o. The fundamental form was e; this e could appear as o under certain conditions, and in other conditions both e and o could disappear entirely. On this basis we speak of given forms in Indo-European as exhibiting, respectively, the e-grade (or full grade), the o-grade, or the zero grade. The e and the o might furthermore occur as long e or o, termed the lengthened grade. =====