Taxi Driver missions involve picking up passengers and driving them to their destinations within a time limit. To start, enter a Taxi and press the designated button. You'll earn money for each successful fare, and completing a certain number of fares (100 total) will unlock the 'Boost' ability for all taxis, allowing them to jump. This is more of a fun unlock than a critical one, but it's a good way to earn early game cash.
These missions are a good way to familiarize yourself with the city's layout and practice your driving skills. Be careful not to damage your taxi too much, as passengers will refuse to pay if the vehicle is too beat up. The 'Boost' ability can be quite entertaining and useful for shortcuts once unlocked.