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A Low-dropout regulator (LDO) is a device which will regulate a stable voltage as long as the input voltage is "somewhat" higher (depends on the exact regulator) than the required output.
It is important to know and understand that an LDO does this by "burning of" the excess voltage and this can and does result in quite a lot of heat which must be handled in the design. The amount of energy burned off is easy to calculate. Let's imagine using a 3.3V regulator from a 5V source. The voltage drop over the regulator is 5V - 3.3V = 1.7V. If using 100 mA that would result in 1.7V * 0.1A = 0.17W (or 170 mW) of power.