"Vested" in a pension plan means you are entitled to a pension, usually at Normal Retirement Age, which is often age 65. If you later become eligible to begin receiving your pension before age 65, the monthly amount will be reduced. The earlier you begin, the more the reduction.
Vesting means you now have a legal right to your future pension benefit, although it will be a small amount, unless you continue to accrue Pension Credit in your pension fund.