This will usually be the amount you pay for calls, texts and internet usage on your mobile phone.
If you choose pay-as-you-go, this means you pay for credit to your mobile to use for calls and texts. When it runs out you have to purchase more credit.
Look at your bank statement and see how much you have spent on credit in the previous month or two.
If you have a mobile phone contract then you pay a set monthly fee for a fixed period of time and have a monthly allowance of calls, data and texts. Before you sign up for a contract you will be told how much this is going to cost you per month. If you have one set up already and don’t know how much you’re paying, get in touch with your mobile phone provider and ask or check your bank statement.