The anterior interval is a gap between the tibia and the patellar tendon just at the top of the tibia.
This illustration is of the knee joint - cut down the middle.
The anatomy here is a bit complicated. The capsule (with some fluid in it) is marked in blue. The yellow structure is the fat pad which fills much of the region behind the patellar tendon. The menisci are shown schematically in white. A number of lubricating pockets behind and in front of the patellar tendon allow the patella to glide as the knee bends and straightens.
The anterior interval is a gap that allows movement of the structures behind the lower patellar tendon. Normally it can be easily probed. But if there are problems in the knee sticky adhesions in the anterior interval can 'gum' the patellar tendon down to the bone, and make movement painful and restricted.