Your bill cap covers costs that are not included in your normal plan such as picture messages and calling/texting international/premium numbers (from any country). The amount you'd be charged for any chargeable calls/ texts is limited by your bill cap, so if you have your bill cap set to £0 you won't be able to make any calls/ texts that are not included in your plan. You can see a list of the costs for calling/ texting these numbers here.
If you're not sure where a number is a premium number then you can check if you'd be charged for calling it by clicking on the check call price option in the help section of the Honest app.
Your bill cap does not cover calling and texting UK numbers from outside the UK and EU or using data outside the UK and EU. We have a separate daily roaming cap for that called World Travel. So if your bill cap is set to £5 then you can only be charged a maximum of £5 for premium calls or texts, but you can still be charged more for using your phone to call UK numbers and use data when abroad. You can see a list of our World Travel costs here.