This depends on the kind of passes are on the members account.
When free passes are assigned as part of a plan (Setup » Plan » Edit Plan » Time Passes tab), they do NOT roll-over, which means that if you had time passes that you didn't use in the first week/month, they will not be available for you to use in the following week/month.
Cobot will assign a new set of free passes every week/month as selected. However, an admin can assign free time passes to give a member the passes they didn't use in the period before the membership renewed. Passes assigned this way do NOT expire.
When setting up plans, it's possible to indicate an expiration date for time passes (under the Time Passes tab). For example, if you set 30 days, time passes will expire 30 days after their date of purchase. If the field is left is blank, time passes will not expire.
When members are being checked in, whether manually or automatically, Cobot will automatically try to use the passes that expire first. To follow the examples above, if you have two passes that renew each week, Cobot will keep using only those until you check in the third time in the same week. At that point, Cobot will use one of your 30-day passes you've purchased.