Every pound from this source is taxed at 20%

What each part means

The S at the start means HMRC has you down as living in Scotland, so Scottish income tax rates apply to your pay or pension.
BR means all of the income from this employer or pension is taxed at the Scottish basic rate of 20%, with no tax-free amount.
HMRC normally uses BR for a second job or a second pension, because your Personal Allowance is already being used against your main income. If this is your only income, it is likely wrong.

Only right if this is not your main source of income.

HMRC uses this when your Personal Allowance is already used against another job or pension. If this is your only income, ring HMRC: you are almost certainly paying too much.
If it is a second source, check that the rate matches your total income. A D0 or D1 code when your total is under the higher rate threshold means too much tax is being taken.

1
Sign in to your Personal Tax Account. Under PAYE you can see what HMRC thinks you earn from each source and what is in your code, and you can correct most of it yourself.
Personal Tax Account (GOV.UK)
2
If you would rather talk to someone, ring the Income Tax helpline on 0300 200 3300 with your National Insurance number to hand. Ask them to explain each line of your code and to send you a P2 if you have not had one.
Income Tax enquiries (GOV.UK)
3
Once HMRC changes your code they send the new one to your employer or pension provider. Any tax you overpaid earlier in the year comes back automatically in your next pay on a cumulative code.