Hello and welcome to MedHelp and our forums. We appreciate your question and wish the best for you during your upcoming surgery. We're glad you proactively sought help and are being cared for by a doctor who will be performing the Mohs Surgery. What type of skin cancer do you have?
A surgical margin is the entire area to the edge of the tissue being removed. It will run wide (what you see on the skin) and deep (how many layers down).
Mohs surgery is really the gold standard for treating two of the three skin cancers. I'm assuming you have one of those two, basal or squamous cell. The doctor performing surgery will remove layers one at a time until there is no cancer present. It's very precise and removes the cancer while doing as little damage as possible to the healthy area around it. Typically, they will remove a small amount outside of the skin cancer lesion extending the margin a bit but as little as possible if that makes sense. Here is more information https://www.mayoclinic.org/tests-procedures/mohs-surgery/about/pac-20385222.
Feel free to ask your doctor but would imagine that this notation is just signaling that the margin is set circumferential peripheral (what you see on surface) and as deep as it needs to be and they will likely remove layers to that point. Mohs surgery allows doctors to have complete margin control for removing skin cancer lesions.
When is your surgery scheduled?
I really do understand how you must be feeling. Cancer is always worrisome. Mohs surgery has a high success rate. Please do tell your doctor about the pain. Perhaps they are talking about the biopsy but remember that in Mohs surgery, they control the margins precisely to get all of the cancer with as little removal of other tissue as possible. I would imagine this especially important with the location being on your forehead in the center. Typically basal cell is fairly superficial and doesn't extend very deep into the skin. Also keep in mind that Mohs surgery has a 99% cure rate.
Here's another article that includes information on Mohs surgery. https://www.skincancer.org/skin-cancer-information/basal-cell-carcinoma/bcc-treatment-options
After care will include also watching for future lesions. You will likely have regular checks from your dermatologist to ensure early identification of future lesions. And they now make a topical cream that often works very well at that stage.
You have a little time until July 19. Again, please let your doctor know that you are experiencing pain.
You are absolutely right, sunscreen is essential and hats too. Prevention is so key. We are glad it is BCC as well and not melanoma. Let us know how things go with the Moh's surgery!