When you are given a tax code it will consist of some numbers followed by a letter.
The number will indicate how much personal allowance you can have in the relevant tax year and the letter gives you more information about how that number was arrived at.
In the case of the letter L, this is the most common letter used as it is given to those people who are entitled to the standard personal allowance.
If you are assigned the letter L at the end of your tax code then it is likely that you have pretty simple tax affairs.
In order to work out the amount of personal allowance that you are are entitled to you will need to multiply the number by 10 to get the actual figure.
So for example, if your tax code is 1150L then you multiply 1150 by 10 and you get a personal allowance of £11,500 (which is the standard personal allowance for the tax year 2017/18).
If you believe that your tax code is wrong then you can ring HMRC on 0300 200 3300.