Manual power generators provide power to the most basic equipment in need - radios and radio beacons, cell and smartphones, navigation lights and similar.
They provide tens of watts of power and they are not suitable for charging the main boat battery - while able to provide 2-3 Amps, they would require 10 hours to charge the 12V battery with 20-30Ah.
However, if you are unable to start the main engine manually due to weak battery or weak jump starter, recharging it with just 5-10Ah is sometimes more than enough to bring the battery to life, just enough to start the main engine - if that is the case, after starting the main engine, go to the nearest port and don't turn off your main engine until you are safe in the harbor (after which, you will find some good electrician to check your electric system).
Manual power generators can be standalone units or part of the power generators.
These units can be powered by hands or by legs (like bike, for example) using various rotating or pulling pedals/handles.