In terms of chords a "seventh" can mean at least two things.
1) It can be the 7th note of a scale, on top of which a chord is constructed. In the C major scale the 7th note is B. If you build a chord on that with the notes of this major scale, it's going to be a diminished chord. However, in rock music a major chord built on the "flat 7th" is very common. (Bb major, if we're in C). This cannot be heard in the Chuck Berry classics - which are based on the I, IV, V chords (C major, F major and G major if we're in C). It became common later (Kinks, Hendrix...).
2) Like was already pointed out, when you add another note to a triad (a 3rd above the previous note), you're going to have a chord that is called "something" seventh. In "traditional" chords the next note is always a 3rd above the previous one.
So, the "7th" here could mean that the "7th" shouldn't be added to any chord, or it could mean that a major chord based on the "flat 7th" of the scale shouldn't be used. But they used another chord in the chorus anyway that isn't common in Chuck Berry classics as far as I now - a major chord built on the flat 3rd.
Of course, people who didn't know this already won't have a clue what I was just talking about.