The Beneficiaries of a Trust (or a Private Foundation ie a corporatized Trust, which typically also has beneficiaries) are the person/s for whose benefit the Trust or Foundation is established. Put another way a beneficiary is a person who is deigned ultimately to benefit financially from the set up a Trust or Foundation as the case may be.
Trusts (and Private Foundations) are usually designed to ensure that the beneficiaries of the Trust/Foundation receive some kind of financial or lifestyle benefit from the Trust/Foundation and its assets.
Such benefit may be by way of payment/s of income from the Trust/Foundation.
Alternatively the Trust/Foundation may provide for certain Trust/Foundation assets/capital to be transferred to a Beneficiary (or Beneficiaries) at a certain time.