A self-insured retention is a layer the policyholder retains before the policy responds. The key difference from a deductible is that with an SIR the insured typically handles and pays claims within the retention themselves, including defense, before the carrier steps in.
Large SIRs on a sub's policy can be a concern for GCs, because additional insured protection may not function until the sub satisfies the retention, something a thinly capitalized sub might struggle to do.