A codicil is a supplement to a will. If your will is already written and you want to alter it, you add a codicil.
When your seemingly ancient neighbor marries a woman less than half his age, you might notice that his adult children suddenly stop coming to visit. This may be because he added a codicil to his will granting his new wife access to all his riches. Getting your inheritance in writing is a good idea, but a codicil can change everything.