Dogs very commonly get lumps. One of the most common are lipomas. These are fatty deposits that collect under the skin. They are usually soft, smooth and movable. It may very likely be a lipoma however, as with any lump we humans find in our bodies, it is always advisable to have it biopisied.
The next step would be having a vet examine the cells under a microscope. This is done by collecting the cells with a biopsy needle. The good news is that lipomas are usually benign, they usually are soft and they seem to not cause any pain to the dog. However, sometimes they may grow very large and depending on where they are located they may cause discomfort and therefore need to be removed. It is best then to have it checked out, to rule out any possible malignancies such as basal cell tumors, slow growing tumors especially seen in senior dogs which are always a risk. After ruling out a malignancy most vets would recommend keeping an eye on it and keeping track of its growth by measuring it if it does not interfere with movement or is in an area where it would be best to have it surgically removed.
You can read more about lipomas and other types of lumps in the reference link below:
While most lumps are simply not a threat as stated before it is always advisable to have them biopsied by the vet, the saying better safe than sorry is always the best course of action. Sorry this turned out a little lenghty, my best wishes and let me know if i can be of any further assistance!