This exercise is from the Fundamentals section on Playing across strings.

Here we play the good ol' A Chromatic scale again all along the A string, from open string to 12th fret. However, the next time, we play the exact same scale - but across the strings. This means we can keep the hand in 1 position and use all the fingers rather than stick to 1 string and 1 finger. Learning to do this is a vital part of playing guitar. In many cases it is more useful and economical to play across strings. Just have a practice and get the hang of it. Don't worry about speed, just get it right first.

Notice that the notes from both scales are EXACTLY the same, and played in the same ascending order. However, when playing across the strings we play up to the 4th fret and then switch to playing the open note of the next string. We then continue to play through to the next 4th fret and switch strings again. Finally we stop on the 2nd fret of the G string - this is the octave A note, and the same as playing the 12th fret on the A string.

This works because all strings, except one, are tuned so that the 5th fret note will be exactly the same note as the open note of the next higher string. So - fret 5 on the A string is D, which is the same note as the next open string! We talk about this more in the Fundamentals lesson. Just beware that all the strings 'switch' like this at the 5th fret - except for the G string, which tunes to the next string (B string) at the 4th fret - i.e. 1 fret lower than the rest. This may seem awkward, but actually it means that many chords become easier to play - just because of the way our hands are designed!