The 'anterior interval' is the region of the knee between the fat pad and the front of the tibia bone (anterior tibial plateau) and the patellar tendon.
When the keyhole surgeon looks at the anterior compartment from above, the fat pad is attached to the anterior horns of the menisci, and to the intermeniscal ligament. During an arthroscopy, the surgeon can lift up the intermeniscal ligament with a probe, and enter the anterior interval, with the tibia behind.
The anterior interval is one of those areas of the knee that can become locked up with adhesions, resulting in a stiff knee. In this case the surgeon may undertake an 'anterior interval' release, to free the adhesions.