The word rectifier is related to rectification in electrical terms, which means allowing only one half cycle of the input AC or the alternating current and blocking the other half.
As we know an AC signal may be made up of alternating voltage peaks which alters from a high positive peak to zero, continuing from there to the negative peak and back to zero.
This cycle repeats number of times per second, depending upon the frequency of the signal.
For example our domestic AC input may be typically set at 50 hertz or 60 hertz depending upon the country specs. one Hertz meansone cycle or pulse per second.
What is a Bridge Rectifier.
A bridge rectifier is essentially a configuration of four diodes, connected in a such a way
How Bridge Rectfier Functions
Thus, in contrast to a single diode where only a single cycle is processed, using four diodes makes it possible to convert the whole signal cycle of the AC in a single particular order - only positive or only negative, depending upon how you measure it, that is how you connect the multitester at the output.
You can understand the entire the working of a bridge rectifier by reading more about it.