BR means basic rate, which means that you pay tax at the basic rate on the whole of your earnings from that job. At the moment, the basic rate is 22%, but it reduces to 20% on 6th April.
M1 means month 1, which is a method of calculating the tax due when your employer doesn't know how much you've earned previously. It doesn't really have any effect on a BR tax code, anyway. You may also find the M1 part disappears next month, as the new tax year starts then.
BR is normally used for your second job if you have more than one. If this is your only job, you need to contact your local tax office to find out why this code is being used.