This is what is called a marriage bond.
If the couple married, then no money had to be paid.
However if the couple did not marry as planned, then a fine was due ... in this case $300.

The person who agreed to pay the fine could be the groom
or one of his relatives
or they would get a bondsman who would be indebted.

The purpose of a marriage bond was to affirm that there was no legal reason,
the couple could not be married (for instance no existing marriage in place).
Also a marriage bond affirmed that the groom would not change his mind.

NICOLE