All the members of the family of instruments in the string section (excluding the double bass) are tuned in fifths. This means that (including the note itself), there are five notes in between each of the strings and when musicians tune up these instruments, they tend to play two strings simultaneously as the perfect fifth is a very pure interval.
When composers or string arrangers write fifths, there are a few things that need to be borne in mind to make it feel more natural to the players. One example of well written fifths is in Vaughan Williams’s ethereal ‘Lark Ascending’ – there is a passage where the violin enters very quietly in a ripple of fifths. The composer had a good insight into the playability of this passage as it falls very naturally under the hand and doesn’t go beyond third position on the E string. It’s all a matter of remembering that on stringed instruments, there are only four fingers available (with three intervals between these fingers) and therefore if a run of fifths is written, a gap of more than four notes between any of the consecutive fifths when written rapidly may cause a problem for all but the best of players.
The trickiest fingers to play fifths on are the fourth fingers because these are generally smaller than the other three fingers and therefore can find it more challenging to bridge the distance between two strings (this is particularly the case on the violin between the G and D string as it is on the viola between the G and D and C and G strings). On the cello, such fifths might be played avoiding the fourth finger although I do know some cellists with such sturdy hands that they’d probably be capable of anything!
Another thing that can cause problems (again for all but the most virtuosic players) are sustained fifths, written higher than third position on the E and A strings of the violin. Here the strings gradually become more elevated from the fingerboard as they make their way towards the bridge. With a soft hand and a sweeping motion, fifths can still be played in tune at this register but when sustained may not consistently produce the finest sound, so if fifths are needed up this high, a pianissimo marking would be far more successful than asking your players to attempt this fortissimo!
The quality of a fifth is what we call modal. The modes were a system of scales which were superseded centuries ago by our major and minor scales and crudely relate to the white notes on the piano (although centuries ago the distances between these notes were subtly different due to unequal temperament). Fifths can give an ‘other worldly’ quality and were used by composers like Debussy and Ravel to give a nebulous and dream like feel to a piece. A clever use of fifths can also evoke a slightly archaic feel as they were such a mainstay of 13th and 14th century harmony.