What the pattern says

• (?<year>start of a group captured as "year"
• \d{4}any digit, 0 to 9, repeated exactly 4 times
• )end of the group
• -the character "-"
• (?<month>start of a group captured as "month"
• \d{2}any digit, 0 to 9, repeated exactly 2 times
• )end of the group