Most thumbnail text fails because it was designed at full size and never checked at phone size. These rules keep your words readable everywhere.
Go big and bold
Use heavy, condensed, or extra-bold typefaces. Thin fonts evaporate when the image shrinks.
Three words is the sweet spot
The fewer words, the larger each can be. Cut your phrase to its most compelling core.
Add a stroke or shadow
A thin outline or drop shadow separates text from any background, so it stays legible over busy images.
Mind the contrast
Light text on dark areas, dark text on light areas. Never place text over a region that matches its color.
Keep it out of the corners
Leave margins so nothing clips, and avoid the bottom-right where the duration badge sits.
The squint test
Shrink the thumbnail to the size it appears on a phone, then squint. If you can still read it, your viewers can too.