This is a good question - Your pension scheme should have sent you information and there should be nothing to worry about if it is well known master trust
the following explanation has been taken from a very help article at - https://www.aviva.co.uk/business/workplace-pensions/corporate/trust-pensions/aviva-master-trust/whats-a-master-trust/
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Very briefly, it’s a defined contribution, occupational pension scheme which is set up under trust. Master Trusts are set up by a scheme funder, which is the company that provides financial support to the scheme.
The scheme’s aim is to provide a workplace pension that can be used by multiple unrelated employers. This contrasts with traditional OPS schemes that are set up to provide a workplace pension for a single employer, or a group of employers that are part of the same group of companies.
While some master trusts have been set up to serve the automatic enrolment market, and small employers in particular, this isn’t the case for all master trusts. There’s a wide range of master trusts in the market and it’s likely there’s one suitable for most employers.
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Generally speaking a Master Trust should reduce your pension costs and provide some good fund choices
