I looked up the amino acid sequence of lysozyme here: http://www.biology-pages.info/L/Lysozyme.html

Then I crossed referenced that with the lysozyme sequence on UniProt: https://www.uniprot.org/uniprot/B8YK79

My question is why does the sequence on UniProt start with eighteen other amino acids before getting to the start of the sequence described in the first source?

The sequence on UniProt starts with "MRSLLILVLCFLPLAALG". I know the M comes from the start codon in the gene for the protein, but where does the rest of this come from?