Multifocal IOLs contain more than one focusing power within each lens (similar to bifocal glasses but without any line or distortion).
They are designed to help you focus on the far distance, at middle range (for using a computer screen), and up close (for reading) without needing any glasses.
Multifocal IOLs work well in daylight. In the dark, they can make you see halos around lights. This can cause glare and interfere with night vision.