What purpose does the small cylindrical module near the end of my laptop charger cable serve?

It is a passive electrical device that helps to isolate the laptop from electromagnetic noise, either from waves picked up by the wire (which acts like an unintentional antenna) or from existing noise in the upstream AC line or AC-DC converter. Although it probably is not necessary in everyday usage, it allows the laptop to pass FCC tests for being able to accept a certain amount of environmental EM noise.