A meniscus tear can be degenerative and occur with microtrauma over time, or it can be the result of an acute trauma. In neither case is surgery an absolute indication.
The majority of meniscus surgeries these days are performed arthroscopically (with a small camera inserted into an incision). There are two main types of meniscus surgeries: a removal of part or all of the meniscus (called a menisectomy) or a repair.
The meniscus primarily provides support and shock absorption to protect the knee, and has a secondary stabilizing effect. Because it is not a primary stabilizer, it is not always necessary to perform a surgical procedure. Often times if just a meniscus tear is suspected in the absence of other ligament damage, it is recommended to wait at least six weeks and see if you can heal conservatively before attempting surgery. Surgery can be invasive and lead to arthritis in the future, so if you can heal conservatively and return to your prior level of function painfree, it is typically better for you in the long run.
If you suspect a meniscus tear but there was no acute injury, you may want to see a health professional to rule out other pathologies. If it is a meniscus tear that has occurred overtime, conservative care should be your first option. A recent study in the New England Journal of Medicine found that in a large sample of patients with meniscus tears and the presence of osteoarthritis (this is most often a degenerative tear) were randomized to either physical therapy or surgery, the outcomes for those who remained in their respective groups were not significantly different at 6 months and 1 year. Further research needs to be done in order to conclude that surgery and conservative treatment result in the same outcome, but the important message of this study is that patients can be just as successful when they undergo physical therapy alone to address a meniscus tear as if they were to undergo surgery.
In the event of an acute tear where you are unable to fully bend or straighten the knee, it is painful and swollen, and there is a clicking or popping, you should get it checked out either by a physical therapist or an orthopedic physician to rule out possible ligament damage other than the meniscus.