Apparently you are using a Mac laptop or compact keyboard. Several generations ago Apple took control of the Function ["F"] keys for their own purpose. As others wrote the default setup requires using the fn key in unison with the indicated Function key in order to perform keyboard shortcuts other than the Apple assignments.
To reverse the behavior go to > System Preferences> Keyboard - Keyboard to select the option for standard function keys.
See the Mac Help article Use keyboard function keys on Mac or visit this Apple KB **** for more details on use of the laptop keyboard.