In more detail
The classic look uses the Impact typeface in capitals, white with a thick black outline, one caption at the top and one at the bottom. The outline is what makes it work: whatever the photo behind, either the white fill or the black edge contrasts with it. Many variations exist, such as captions above the image on a white band, or labels placed on parts of the picture.
Why it matters
Readable memes keep the caption short, high-contrast and clear of the picture's focal point, and large enough to survive being shrunk in a feed. The base images are usually someone else's photos or film stills, so whether reuse is allowed depends on the source and the country's rules on parody and fair use. When posting, add alt text with the caption, since screen readers cannot read text inside an image.
Example
White text over a pale sky colour #dfe9f3 has a contrast ratio of only 1.23:1, which is almost invisible. The black outline against the same sky reaches 17.09:1, so the letter shapes stay legible even where the fill disappears.