Simple questions can have simple answers but also get complicated to answer fully. Yours is one. BarracudaHockey's answer was only one word. Short, sweet and correct. But it does not tell you all you need to know about flying in Class G airspace or what the limits of the Class G airspace are in your area. There are several classes of airspace which are explained in The Recreational UAV Safety Test (TRUST). You can take the TRUST here: https://trust.modelaircraft.org
The FAA requires you to register your Drone/Unmanned Aerial Vehicle if it weighs over 250 grams ($5 for 3 years). If you fly models weighing more than 250 grams, then the FAA also requires you to take the TRUST course and test (which is free) and then print out and keep the completion certificate as proof of completion.

You can get more information about the airspace restrictions on model aircraft where you are flying at: https://www.uasidekick.app/#/map/ama
Please note that the increased altitude waiver up to the top of Class G airspace is only for valid for certain designated locations... mostly AMA sponsored Federally Recognized Identification Areas (FRIA's).

There are several classes of airspace: A, B, C, D, E, F and G which are more fully explained here: https://pilotinstitute.com/airspace-explained/

A short version is:
Class A (think Altitude) is that airspace between 18,000 and 60,000 feet where all aircraft must be on an Instrument Flight Plan clearance controlled by the FAA 's Air Traffic Control (ATC) system.
Class B (think "Big") is that airspace surrounding specified large airports with high jet traffic concentrations (such as Atlanta, Memphis, Charlotte, New York, Los Angles, etc) Class B airspace requires a clearance by the ATC prior to operating in Class B airspace.
Class C (think Communications) is that airspace around medium to large airports where two way communications must be established with ATC prior to operations in Class C airspace.
Class D (think "Dialogue") This is typically airspace from the surface up to about 2500+/- feet around smaller airports having an operating a control tower. The radius of Class D airspace is normally about 5 miles from the airport.
Class E ( think Everywhere else) is controlled airspace that requires 3 miles of visibility and 1000 ft cloud ceilings for visual flight rules aircraft operation. With some exceptions, Class E airspace generally starts at 1,200 feet above ground level, but may start at the surface of some airports or at 700 feet a few miles from an airport. Radio communication is not required for visual flight rules operations in Class E airspace.
Class F (think "Foreign") The US does not have any Class F airspace, but a few other countries (the UK and Germany) do. It is hybrid mix of Class E and Class G where ATC can provide non-mandatory clearances.
Class G (think "Ground") This is uncontrolled airspace starting at the surface and extending upward to the base of Class B, C, or E airspace, which ever is lowest. Visual Flight Rules air operations require 1 mile visibility and aircraft must remain clear of clouds. No radio communications are required. Class G is not everywhere because Class B, C, D and E airspace can start at the surface, which excludes Class G.

Of course the actual shapes, floors and ceilings of these classes of airspace vary considerably and are depicted on Aviation Sectional charts which can be down loaded free at: https://www.faa.gov/air_traffic/flig..._products/vfr/