Put simply; a rectifier changes AC electric current into DC electric current.
There are several designs increasing in complexity and parts, but all are based on four or more diodes and optionally a capacitor.
single phase rectifiers
3 phase rectifier
Here’s one I prepared earlier:
red labeled wires positive, brown/black labeled wires negative, 3 other wires green individual AC phases.
Rectifiers produce some heat while they are operational and require some cooling. Normally cooling is done by adding a passive cooling block, thought fans may also be used.
Rectifiers need to be matched for the application based on their voltage and maximum current.
The rectification losses are not large when compared to the reverse of the process; inversion.