Week returns the numeric week (1-53) of a date depending on what day you choose starts the week. Different folks start weeks on Sundays, Mondays, etc.
If you set Startday to be 1, Monarch assumes weeks start on Sunday, 2 is Monday, and so on up to 7 for Saturday.
From the Help page on the function: "Week #1 of each year is assumed to begin on the first occurrence of the designated day. Any date that falls earlier in the year is assigned to week #0." Please note that this definition may or may not coincide with international standards for US or European weeks.