A beneficiary is the recipient of a gift in your will. Beneficiaries can receive either a residuary gift or a specific asset or sum of money (a legacy).
Anyone can be a beneficiary under your will. If someone is under 18 (or any older age you have said you want them to inherit) when you die the executors will hold onto the asset and look after it for your beneficiary as a trustee until they reach 18 years old (or such other age you have specified).