Matthew Champion, in his book Medieval Graffiti: the lost voices of Britain's churches (2015), aims to throw light on the numerous carvings that can be found on churches throughout England. 
The definition of graffiti is ‘writing or drawings in a public place’ (Oxford English Dictionary, 2012),  and in Medieval times it was common practice to inscribe symbols or writing into the fabric of buildings such as churches,  castles and private homes. The church though is where most of this graffiti can be found, and Champion has detailed his findings from surveys of various churches throughout the country,  with the main focus on the East Anglia area where he is based.
Champion (2015) writes that graffiti is a practice that was acceptable until around the 19th Century when it was then deemed to be destructive, and as we view it today, an anti-social act to write on buildings, but until then it was commonplace, with even the clergy themselves leaving their own personal marks. 
His findings include simple memorial inscriptions, ritualistic symbols, and also more elaborate  drawings, probably done with tools such as a compass by craftsmen.
The book includes chapters on each type of graffiti found and the author explains the theories attributed to their meaning and why some types of graffiti are more prolific in certain areas of the country, also why they are found in large numbers in particular areas of the church.
Champion's interpretation and dating of the graffiti uses a combination of historical fact, parish records, and sometimes common folklore,  but he also references the only other work on graffiti in English churches which is Violet Pritchard's publication on English Medieval Graffiti (1967).  He uses many of her findings  as a basis for his research, but in some cases he has his own interpretation of specific types of graffiti which he attributes to more recent historical findings and changes in academic thinking since her book was published.
Champion (2015) writes that most of the graffiti was produced by the poorer members of society  as the more affluent in the community could afford to have memorials and monuments constructed including carvings, stained glass windows, and headstones, but the majority of people could only record their lives through the many types of inscriptions they scratched into almost every surface of the church from the porch right up to the bell tower.  Champion(2015) describes how the decorations in the church  would have originally been vibrant  and full of colour, in fabrics and in paintings on the walls, and this would have made the perfect backdrop for the various writings, drawings and symbols etched into the surfaces as they would have been highlighted against the colour.
Graffiti is something we find hard to accept nowadays as we view such  actions as vandalism, but hundreds of years ago it was their version of Facebook, the social media of the time: a place to record hopes and fears, to remember those long gone, and to celebrate the achievements of the humble, not just the grand.




References

Champion, M. 2015. Medieval Graffiti: the lost voices of England’s churches. London: Ebury Press.

Pritchard, V. 1967. English Medieval Graffiti. Cambridge, Cambridge University Press.