Thumbnails matter more than their size suggests. A thumbnail is often the only part of a video a person sees, and the difference between one that gets clicked and one that is scrolled past comes down to legibility at a very small scale.
How it works
Every YouTube video has thumbnails stored against its video ID, at several sizes. The ID is the part of a URL after the equals sign in a standard link, and it is the key to all of them. Give the tool that ID and it can fetch each stored size, rather than trying to grab the image from the page, which is where this usually goes wrong.
Getting the best available version
Quality depends on what was uploaded. A creator who supplied a high resolution thumbnail has one stored at a large size, and that is what you want. Videos where the uploader did not set a custom thumbnail have a generated one, and these are frequently much smaller and softer than a deliberate upload.
So the honest answer to why a thumbnail looks blurry is usually that no larger version exists rather than that the retrieval failed.
Using thumbnails well
If you are studying thumbnails for your own channel, the interesting comparison is legibility at actual size. Most people view them at roughly three hundred pixels wide, which is far smaller than the images look on a desktop. A thumbnail that reads as clear on a large screen can be completely unclear at the size anyone sees it.
Text is the most common failure. Two or three short words read well; a sentence does not. And the most reliable pattern is still a close face and a high contrast, because that reads correctly at any size.
A note on using other people’s thumbnails
Retrieving a thumbnail is straightforward because YouTube serves them openly, and that does not make them free to use. A thumbnail is part of someone’s content and carries the same copyright as the video, so it needs permission. It is entirely reasonable to study what works and entirely wrong to publish another channel’s image as your own.
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Reading thumbnails at the size people see them
The most useful thing you can do with a set of thumbnails is look at them small. Most of them are viewed at roughly three hundred pixels wide, which is a fraction of a desktop display, and a thumbnail that reads clearly when you download it can be completely unclear at the size it actually appears.
Shrink it until you can no longer read the text, then look at what is left. That is a reasonable approximation of the moment someone decides whether to click, and it reliably shows which thumbnails are carrying their message in shape and which depend on a word you cannot see.
What the strongest ones have in common
Across most categories the recurring pattern is a face taking up a reasonable share of the frame, high contrast between subject and background, and two or three words at most. The face is doing most of the work, because a human face is what the eye is quickest to lock onto.
Contrast is what stops the image blending into the page around it. A thumbnail with a busy background loses to the recommended videos beside it, whatever the subject is.
On text, the discipline is to write the words you would say out loud. Short, large, and left-aligned in the safe area, since the bottom right corner gets covered by the timestamp.
Testing your own before you upload
It is worth looking at your thumbnail next to the ones it will appear among rather than on its own, because the comparison is what the viewer does. If yours is smaller in the frame or lower in contrast than everything around it, it will be skipped regardless of how good it looks alone.
And if you have changed the title, the thumbnail and the first few seconds, you should probably upload to a small audience first and check the click-through before committing to it.