Supposedly, it has to attract enough attention for someone to click on it. This is of course vague, but then again judging thumbnails is subjective. I think the thumbnail should correspond to the title, and have something to make it stand out, whether the art or writing on it.
Creating an eye-catching thumbnail is crucial for grabbing viewers' attention and enticing them to click on your content. To craft an attractive thumbnail, consider these key principles. First, focus on clarity and simplicity. Your thumbnail should be easy to read and understand at a glance. Use bold, legible fonts, and avoid clutter. Keep it concise; less is often more when it comes to thumbnails.
Next, pay attention to color and contrast. Vibrant colors and high-contrast elements can make your thumbnail stand out from the competition. However, ensure that your color choices align with your overall branding and the tone of your content. Additionally, incorporate relevant imagery or icons that represent the core of your video or article. This helps viewers quickly grasp the content's subject matter. Lastly, maintain consistency in your thumbnail style to build brand recognition over time.
From what I've learned from content creator tips.
1. don't put too much words on it, use "easy to read fonts"
2. watch out for colors that are not to "crowded" or bright
3. don't overdo the art or pictures you put in it
4. It must reflect the content of your video through simple catchy image or even catchy words