If your tax code is on a week1/month1 basis it means that when your pay is calculated, any previous pay you have received and any tax you have paid are not taken into consideration.

Normally, tax is calculated on a "Cumulative basis" which means that each month/week you receive an extra month/weeks worth of freepay and 20% tax bracket allowances and these are added to all all the previous allowances you have received for each pay run since the beginning of the tax year on 6th April. All your pay is added together from that date too and the tax due is then calculated. Then the tax that you have already paid is deducted and what is outstanding is then taken from the current payroll. This means that the tax is always correcting itself to make sure you don't over or under pay tax by the end of the financial year.

This does not happen on a week1/month1 basis. This could be for many reasons, one being that you have underpaid tax earlier in the financial year and HMRC have increased you code to allow for this. The week1/month1 basis will prevent you paying back the underpayment in one lump and allow you to spread it over the remaining payrolls for that financial year.