An executor is the personal representative of your estate. They are the person in charge of taking control of your assets, paying off any debts, and distributing assets to your beneficiaries per the terms and conditions of your will. You can choose anyone to be the executor of your will, but it is a good idea to choose someone who is both competent and trustworthy. The person you choose to be executor should be outlined in your will.
Someone you appoint to be the executor of your will has the right to refuse, so you should have a backup executor in place just in case.