It is a rental charge based on the gross income of the tenant rather than a fixed monthly or annual value.
That means the tenant will pay only base rent until they have an annual income greater than $240,000, although they may agree to some other breakpoint value as part of the lease negotiation.
If their income surpasses the breakpoint, the percent rent is only the portion that has not already paid.
In these cases, the landlord can offer a lease with a base rent that is far below fair market value, but still within the tenant's capabilities to pay during the "lean times".
In this case, the tenant may need a very large space, like a warehouse or factory, but does not expect to see profits for an extended period of time.