When you create a new reel, it gets assigned a unique URL that looks something like "rcrft.co/reel/4449723d-b9e7-4d48-8202-ba2eef91e2d1". You may prefer this since it's harder for someone to guess at what reels you might have, but in some cases you might want to make it easier to print somewhere, or easier to recall. Personalized URLs are great for this reason.
The first time you attempt to add a personalized URL you will be prompted to choose a nickname for your reels. This nickname will be in all of your personalized URLs moving forward, so be sure to pick something that is not specific to a particular reel. This nickname cannot be changed, so your name or business name is always a good option.
You can change the personalized URL for each reel at anytime, but be sure to notify anyone with the previous URL of the new link. The auto-generated URL will never change, so you can share that with anyone who may need unlimited access to the reel in the future.