FiveM also ships with a customized singleplayer mode. This brings the FiveM experience to singleplayer. For this to work, you currently need to have a valid savegame from the original game. Save your game at least once before starting FiveM SP, and copy it into the FiveM profile directory in Documents\Rockstar Games\GTA V (the one with a fivem_set.bin).
FiveM allows the use of certain plugins. You can install these by going to your plugins folder within FiveM's application data folder. If you have no such folder yet, you can create it. Within this folder you can place many types of .asi scripts you would typically use in singleplayer.
Are you running into issues? Read the dedicated troubleshooting manual.