LDO is meant by LOW DROP Regulators.
Regulators gives fixed output voltage when the input voltage is greater than the output voltage.
For example, lets take 78L05 Regulator which is quite popular. This regulator will give fixed output voltage with minimum input of 7V. If you want fixed +5V output voltage, your input voltage should be greater than +7V. In this case, Drop across regulator is +2V, which is quite high in some applications and this drop causes the regulator to heat.
In the case of LDO, take for example, LP2950 which is a LOW drop regulator. Drop across LP2950 is about +380mV. which means that the output voltage will be fixed +5V, when the input voltage is reduced upto 5.380V.
Is this clear to you?
Best wishes