Tesco currently operates a 'defined benefit' pension scheme where (irrespective of the fluctuations of financial markets) the amount someone will receive as their pension is
guaranteed (based upon the number of years that an employee has paid into the scheme and their final salary).
http://www.which.co.uk/money/retirement/guides/company-pensions-explained/defined-benefit-and-final-salary-pensions/
They'll be switching to a 'defined contribution' scheme where money is paid into a fund which is then invested, with the amount of pension at the end being dependent upon how well those investments have performed.
Such a pension system ensures that the employer can't be left having to fund a shortfall (as can happen with a defined benefit scheme) but means that there's less certainty for employees about how much pension they'll get.