Having everything 20/20 basically means, you did a good work on your SEO. It's an indicator which is indicating that you have done all the necessary things to make sure your targetted keywords are in your descriptions, titles and in your tags or you can say it is a check-list which says you have used your keywords in proper ways in your title , tags and in your description and it might eventually help your video to get discovered or help with the impresisons.
Now, again, this doesn't mean you will start getting views or subscribers If you have it on 20/20 or have a 100 SEO score. This SEO score will solely depend on the keyword you chose. If you have done your keyword research properly + did a good job when it comes to SEO + appealing thumbnail then it will definitely help your video out, but alone, SEO won't matter much, being quite honest with you, even If it is at 50% or even less but If you have done well in your keyword research and thumbnail, your video will eventually see the results, it's just that SEO just boost things up a bit more thus, chances of your video getting impressions increases and when your video gets the impression, eventually other factors such as good thumbnail, which will increase in CTR will help you to get views, thus, YT will start suggesting the video more and we all know what's next! yes, a good views which will eventually turn into subs!
Total tag characters here basically mean the tags which you used in your title, description and in your tags. You need to make sure that you use a natural way to utilize all the tags which you feel will help your video here, each tag needs to be used and well placed. It's quite easier to get the first two, which are "
Tags in Title and Description and Tags in Title".
Basically for
Tags in the Title and Description - You need to make sure that the tags or the keyword which you used in your tiles are mentioned in the first 200 characters of your description as that's what will be shown to people when they're on their YouTube homepage or when they're looking up for that specific keywords, thus, it helps you in SEO hence, it's quite necessary to make sure that you input those keywords which you have in your title.
Tags in Title - Now this involves what we have discussed prior, you need to have your targetted keywords in the title of your videos and at the same, you have to make sure that you add that #keyword in your descriptions. It really helps when it comes to SEO as sometimes people use those hashtags with the keywords to find content, so having those will help you to get 20/20, at least that's what I believe.
The last two are quite simple - Just make sure that you insert or input as many tags or relevant tags/keywords as possible in your tag sections, the character limit is 500, so why not make sure that you make proper use of it and the last one,
Title words in your descriptions, basically you need to write your descriptions in such a natural way that your title is added in your description, you need to make sentences or form in a good way, again, this will just help your SEO, however, it's fine if you don't have each of these on 20/20, you should always try to get out off 20 here, but If you can't there's always a room for improvement and you can work on it.
P, S Correct me If I'm wrong here or you wish to add more to what I said. I'm no expert either and we are all learning and growing together!