What is shield money in the medieval times?

Answer 1

Scutage (scutagia in Latin) or shield-money was paid instead of providing military service. Since all knights owed service to someone above them in rank this was a way of avoiding that service, effectively by paying a fine instead.

Scutage seems to have been introduced to England by the Normans as a consequence of the feudal system; it enabled substitutes to be hired in place of people who could not or would not present themselves for military service. The amount involved exactly reflects the pay given to a knight during his 40 day's military service: 8 silver pennies per day.