This will be a hard one to answer without actually seeing the lump, but I can give you a few possible ideas of what could be going on.
The most common reason to see a lump on a dog is something called a lipoma or a fat lump. These are nothing to worry about. They are generally relatively soft, covered in hair and usually moveable. They are not painful to manipulate. They are usually regular in shape (i.e. not covered in a bunch of bumps) but I have seen some that are irregular as well. They are usually underneath the fur and don't affect fur growth at all.
The area you have described could possibly be a submandibular lymph node (sort of like the glands in our neck). However, it doesn't sound like a swollen lymph node because you wouldn't be able to move it around. If lymph nodes are enlarged they are hard. Lymph nodes can be swollen because of infection or cancer. Again, it doesn't sound like this if it is moveable.
Another thought is an abscess. Again, however, these are not usually moveable. An abscess is usually hard and may be a little painful, but they are not always painful.
If the lump feels fluid filled it could be a cyst. Most of these are benign.
It could also be an insect sting or a bruise.
Then the next worry is whether it is cancerous or not. There are many different types of cancerous lumps that can appear on a dog. Usually the cancerous lumps look "ugly". They are often ulcerated, bleeding and purple in colour.
Unfortunately the only way to know for sure what this is is to have your vet take a look. If you think it is a lipoma that I described above then there is no need to rush. You can have your vet check it out next time the dog is in, or make an appointment if it looks like it is growing or becoming bothersome.
Your vet will likely do a fine needle aspirate test which most dogs handle really well. They will put a small needle into the lump and look at some cells under the microscope. If the vet is concerned about cancer they may send a slide to a pathologist to determine if it is something to worry about.
I hope this information helps!