This is the money that an employer pays into an employee's pension. This applies to most people who are paid a traditional salary (i.e aren’t self employed). Employers usually only make a contribution if employees also agree to make one.
Usually these contributions appear on payslips as ‘Employer/E’er pension contribution’ & ‘Employee/E’ee pension contribution’. Employers usually hold both of these contributions back from what they pay into your bank account each month, and pay them directly to a pension provider.