This was first introduced by King Henry I (monarch from 1100-1135). It was an opportunity for Knights to opt out of their duties to fight in wars by paying a tax known as Scutage Tax.
Under King Henry and his successor King Stephen (reigned 1135-1154), this was deemed as reasonable as the tax was not excessive, so those Knights who did not want to fight didn’t have to as the could afford to pay the Scutage Tax.
This was a different story when King John took over the reins. He deemed Scutage Tax as a “Cowards Tax” and slapped a 300% increase on the tax. This was not well received as this forced Knights to fight in wars as they could not afford not to.
It was the actions of good ol’ King John that is believed to have led to the creation of Magna Carter in 1215 and therefore limiting the King’s power.
In other trivia, he was a renowned ladies man (probably with the ladies of those poor knights fighting for him).
Scutage Tax remained for a good while after this, although started to become obsolete as a source of revenue in the 1300’s.
The good thing about the modern era, is we can choose to be a conscientious objector and not have to pay tax for it or even go prison for it (ask Jack Straws dad).