Your “focus keyword” is the keyword or phrase you want your YouTube video to rank for.
This keyword should match and mirror the content of your video. Most often one word is not enough for this.
You should only ever use a focus keyphrase once. You can also search for focus keywords that suits best.
Just enter some keyword in the search box on the left and you get suggestions. These suggestions can be added to your tag list.
The one that fits best should be your focus keyword.