Margreet, BRAF refers to a gene mutation.
Here's a definition from the National Cancer Institute https://www.cancer.gov/publications/dictionaries/cancer-terms/def/braf-v600e-mutation
BRAF (V600E) mutation
"A specific mutation (change) in the BRAF gene, which makes a protein that is involved in sending signals in cells and in cell growth. This BRAF gene mutation may be found in some types of cancer, including melanoma and colorectal cancer. It may increase the growth and spread of cancer cells. Checking for this BRAF mutation in tumor tissue may help to plan cancer treatment."

Did they check for this mutation when you were diagnosed?

That is the type of cancer ! Stage 4 aggressive