A lump-sum payment is a one-time Social Security payment that you received for prior-year benefits.
For example, when someone is granted disability benefits they'll receive a lump sum to cover the entire time since they first applied for disability. This period could cover months or years.
If you received a lump-sum payment your SSA-1099 lump-sum section breaks down the payments into what amount was received for each preceding year.
If you don't see that on your SSA-1099, then you didn't receive one.