What does small, non enlarged incidental reactive lymph nodes that are noted on the neck, which are probably reactive, mean on an MRI of neck? Should I be scared about cancer?

We all have lymph .. Nodes, and radiologists see them all the time. The challenge is separating definitely benign from lymph nodes that need to be further evaluated. Although no one can be 100% sure (nothing is in medicine), the description you list in the report (non enlarged, incidental probably reactive lymph nodes) is most consistent with a benign diagnosis. Please discuss your concerns with your doctor.
Monitoring Centers. Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.
Defense! Lymph nodes are 1 of our primary defense modes. The lymphatic system runs along the blood vessels. It 's our internal sewage system & the nodes are way-stations along the route that process the slime for elimination. With infection, inflammatory process or worst case a cancer, the nodes react by sending in the troops (lymphocytes) to seek & destroy enemies. Common in neck (drains mouth floor).
Reading radiology. Reports is not for amateurs. Why was the mr ordered? Probably not to fish for small nodes. Non-specific small nodes can run the gamut to nothing, syphillis, drug reactions, hiv...And yes lymphatic or head and neck cancer. You need a doctor that you trust to work up your symptoms and signs, and not have you submit the reports on health ta.