The accumulation period in a disability insurance policy is the time frame during which your elimination period must be satisfied.
E.g. You have an elimination period of one month and an accumulation period of three months. You break your leg, are out of work a week; go back to work for a month; your leg isn't healing correctly; you have to have surgery to reset it; you are out of work for 3 weeks and not sure when you get to go back. At this point your Disability policy will start paying because you satisfied the one month elimination period during the three month accumulation period.
E.g. You have an elimination period of one month and an accumulation period of three months. You break your leg, are out of work a week; go back to work for a month; your leg isn't healing correctly; you have to have surgery to reset it; you are out of work for 3 weeks and not sure when you get to go back. At this point your Disability policy will start paying because you satisfied the one month elimination period during the three month accumulation period.