A beneficiary of a Will is simply someone who will benefit from the proceeds of the Will. When someone makes a Will, they will name beneficiaries who they want to benefit from it. Even if someone does not make a Will and the Rules of Intestacy apply and these will lay out who the beneficiaries should be according to the law.
When someone dies and you are the beneficiary, this means that you will receive part of their estate which can include assets, savings, investments, property, or possessions. There may be multiple beneficiaries and the Will or Rules of Intestacy will outline what proportion of the estate you will be entitled to. It is the job of the executor to distribute the proceeds of the estate to the rightful beneficiaries