Voice leading is a common songwriting or arranging technique which (traditionally) results in smooth-sounding chord transitions.
To use smooth or ‘proper’ voice leading when arranging a chord progression for an ensemble, write each instrument’s part so that the performers will make the smallest note jumps possible or no jump at all if the note occurs in both chords (“common tones”).
This technique is particularly important when writing harmonies for background vocalists because smaller jumps are easier to hear and sing against a melody.
Soloists and lead singers are expected to break from this principle, since larger melodic leaps provide interest and can make a melody more memorable.