AMS1117 is a positive low-dropout regulator with a voltage drop of 1.2V at 1A current.

AMS1117 has two versions: fixed output version and adjustable version, fixed output voltage is 1.5V, 1.8V, 2.5V, 2.85V, 3.0V, 3.3V, 5.0V, with 1% accuracy; fixed output voltage is 1.2 The accuracy of V is 2%.
AMS1117 integrates internal overheating protection and current limiting circuit, which is the best choice for battery-powered and portable computers.
The AMS1117 series regulators are available in an adjustable version and a variety of fixed voltage versions. They are designed to provide 1A output current and the working voltage difference can be as low as 1V. At the maximum output current, the minimum voltage difference of the AMS1117 device is guaranteed not to exceed 1.3V, and it will gradually decrease as the load current decreases.
The AMS1117 chip is a positive low dropout voltage regulator with internal integrated overheat protection and current limiting circuits. Its fixed output version voltage can be 1.5V, 1.8V, 2.5V, 2.85V, 3.0V, 3.3V, 5.0V, and is designed to adopt 3.3V output is the ASM1117-3.3 chip.
The main functions of AMS1117:
The AMS1117's on-chip fine-tuning adjusts the reference voltage to within 1.5% of the error, and the current limit has also been adjusted to minimize the pressure caused by the overload of the regulator and power circuit.
AMS1117 device is compatible with other three-terminal SCSI regulators on pins, and provides SOT-223, 8-pin SOIC, and TO-252 (DPAK) plastic packages suitable for SMD mounting. AMS1117 parameters AMS1117 basic parameters Output current (A) 1 Output voltage (V) Adj,1.2,1.5, 1.8, 2.5,
2.85, 3.3, 5.0, * AMS1117 Other characteristics Initial error (%) ±1.5 Differential pressure (V) 1.3 AMS1117 Package type SOT-223 TO-252 SO-8
Three-port adjustable or fixed output voltage
1.2V, 1.5V, 1.8V, 2.5V, 2.85V, 3.3V and 5.0V output current 1A working voltage as low as 1V line load regulation: 0.2% Max. load regulation: 0.4% Max. SOT-223 is optional, TO-252 and SO-8 packages
“Is the AMS1117 more efficient than the LM317? If so, how does it achieve that given that they are both linear voltage regulators?”
The GND / Adj pin current (basically the supply current of the regulator itself) can be different. The effect of this current on the efficiency is usually negligible, however for low-power circuitry, it can be significant.
Considering these two parts, the difference is small: both draw around 50 microampers. If the device itself consumes 20 microampers only, it’s probably a good idea to look for a better efficiency regulator to conserve battery charge.
If the GND / Adj pin current is small compared to the load, the efficiency of all linear regulators is Uout/Uin, regardless of the part or circuit details. All linear regulators have exactly the same efficiency.
The microcontroller used in Arduino Uno, Atmega328p to be exact, runs on 5V but the board is already equiped with a voltage regulator AMS1117 where can provide minimum 7V(DC) to maximum 18V(DC) to its input and it will provide the microcontroller with a stable 5V(DC).