Here, the main idea is the following: when the central bank is constrained in its capacity to reduce the short-term interest rate further by the lower bound on nominal rates, it could resort to communication about its intention to keep the short-term interest rate at the current level for some time in the future.
This would help steer expectations in the desired direction, and thereby exert downward pressure on longer-term interest rates. It is therefore an indirect channel to engineer an easing of credit conditions, broadly defined, even though the level of the short-term interest rate remains constant.