Welcome
@Aldrik you found a great source for everything you need to know about brewing and much more. Meads are more like wine than beer although the processes use a lot of the same equipment. And yes you definitely need a Hydrometer. A Hydrometer allows you to take a reading of the specific gravity before, during and after fermentation. The purpose is to know what the sugar content is at the specific times you take a reading. There are guidelines you can follow that will give you the alcohol by volume as well as an indication that fermentation is completed, etc. Other than a fermentation vessel the Hydrometer is one of the most needed brewing tools.
Now that you have that mead working you will learn a lot about the process. You have only begun. We need more info to help you along like, how much honey to water did you use? Did you add any kind of adjunct, fruit, juice, spice etc. Next I would give it until mid July then sample it and take a reading. Document the specific gravity and how it taste and report back here.
And last but probably most importantly, you must Respect the Mead. A well done mead goes down smooth and taste amazing. The alcohol content is high but you don't really notice it when drinking it. But it will kick your arse and leave you with a killer headache if you consume to much.
Cheers and welcome to your new addiction.