Dealing with scales and chords can be a little confusing at first. Chords are constructed by combining notes from scales. The most common chords are major and minor chords which are three note chords. They are built by using every other note in a scale (what we call "thirds"). For instance a "C" chord would use the 1st, 3rd, and 5th note of a "C" scale (C, D, E, F, G, A, B).
A seventh chord is just a four note chord. They call them seventh chords because the new note is the seventh note in the scale. For instance a "Cmaj7" chord would be made from the 1st, 3rd, 5th, and 7th tones of the scale. (C, D, E, F, G, A, B).
To confuse you more, a "C7" chord is what they call a dominant seventh chord which uses a different "C" scale (C dominant scale: C, D, E, F, G, A, Bb) but is still built from the 1st, 3rd, 5th, and 7th tones of the scale.
Now to make things simple again...
When you encounter a seventh chord (or a ninth, eleventh, or thirteenth chord), you can simplify (substitute) by using the three note major or minor form of the chord. This means that if you run into a C7, Cmaj7, C9, Cmaj9, C11, etc., you can simply play a C chord. If you have a Cm7, Cm9, etc., you can play a Cm chord.
I hope this helps!
Thanks... Mike