Hi Angela
He would have attested (ie taken an oath) to serve six years in the Militia. Recruits served about 8 weeks initial training and 6-8 week a year annual training. While they served they received pay as a regular soldier. If he wished to be discharged, without enlisting as a regular, it would have cost him £1-2 to purchase his discharge.
Part of the oath was agreeing to be 'embodied' - ie called out to serve full-time in the event of a war. Although, in Ireland, they were also used to 'assist the civil powers'. For which they also received payment.
Ken