Is this a medicine for joint or back pain, and what can I realistically expect from it?

No — this is a cosmetic product, not a medicine, and it is not intended to diagnose, treat or prevent any disease. It does not act on the joints themselves; a cream applied to the skin works only on the skin. What you can realistically expect is:

• a pleasant warming sensation on the skin during and after application;
• the relaxing effect of the massage ritual itself — circular rubbing over a tense lower back or shoulders is soothing regardless of the product used, and the cream provides the slip that makes this comfortable;
• basic skin care for areas that are often neglected, such as the lower back and around the knees.

The fly agaric extract here draws on a folk tradition of external rubs, but in this formula its role is to contribute to the warming feel during massage — nothing more is claimed. This makes the cream a reasonable choice if you enjoy warming body products as part of winding down after physical load, and the wrong choice if you are looking for a pain-relief medication. If you have persistent or worsening back or joint complaints, that is a matter for a doctor, not a cosmetic cream.