The bits of your cells that actually do things. Proteins are large, complex molecules composed of one or more chains of amino acids. They have a huge range of jobs in the body, from controlling chemical reactions ( enzymes) to providing a cell's internal scaffolding.
A protein's job is determined by the sequence of amino acids that it is made of. This, in turn is determined by the order of bases in the gene that made the protein. So if the sequence of bases in a gene is damaged ( mutated), it can alter the function of a protein so that it can no longer do its job correctly. This can lead to cancer.