These Stevenson Reeves Homebrew Hydrometers for Wine or Beer play an important part in home brewing. Hydrometers allow you to take a specific gravity (liquid density) reading of the liquid in the brew. 

This will be able to tell you when the beer has finished fermenting so you can then barrel or bottle it. Beer kits will give you an approximation as to what the specific gravity reading should be when the brew is finished so that when your hydrometer reads this value you know that it's finished fermenting. 

The hydrometer is an easy way to tell when your brew has finished. There are other ways of telling like waiting for the airlock to stop bubbling but that's not as accurate. 

You will need a Trial Jar to put some of the brew into so you can take the SG reading.