Hi,
Joint effusion is extra fluid in the joint which happens in response to irritation (from injury or infection or similar). It points out that you have had part of one of your menisci removed (the one on the inside edge of your knee joint) and that you have probably retorn what's left of the meniscus also. You have also torn the other meniscus in your knee (the outer edge one). Posterior means towards the back, so this points out where the tears are.
Kathy