If you arrive late, you'll probably have to wait a bit longer, but you will get seated.
If you don't show up within about 45 min, they consider you a "no show" and take walk ups in those spots.
We made 14 ADRs for our trip a few weeks ago. After the first night, we knew we weren't going to make ANY of the rest of them, and I told the concierge CM this, and asked if we needed to cancel them. She said "no, don't worry about it...if you don't show, you don't show. They'll seat another party that is waiting in your place." She actually urged us to keep the ADRs, in case we DID need them (we never did, but we did walk up to 3 other TS restaurants with no waiting=---over Easter week...)