How to use bold text in iOS 12 for iPhone/iPad/iPod Touch

1. Open the ‘Accessibility Settings’: Settings > General > Accessibility. (See the article Open Accessibility Preferences on this website for more information.)
2. Under the ‘Vision’ section, tap the toggle switch next to ‘Bold Text’.
   
3. You will be prompted to restart your device. Tap ’Continue’ to restart the device with ‘Bold Text’ enabled or tap ‘Cancel’ to return to the ‘Accessibility’ settings screen.
   

More information:

How to enable 'Increase Contrast'.