A sticker mod is where a puzzle's stickers are altered. This can simply be to change the colours (for instance, to fluorescent or glow-in-the-dark), or it can be to make the puzzle more attractive, or easier, or harder, or even a different puzzle altogether.

How a 3x3x3 cube might be stickered so as to highlight corners or edges
Other mods have included card suits (plus Ace and King), dice cubes (the HK Now Store once did a Chinese-dice sticker set (red 1 and 4, all others black, enlarged 1, horizontal 2) (now sadly discontinued); Olivér's Stickers do a Western-dice set (all numbers black, 1 same size as all others, diagonal 2)), various flags, the countries of the world (an atlas; this one is usually combined with a spherical shape mod), and picture cubes (often commemorative, or having the issuing company's logo on one or more sides).
One early mod (sold as a set of stickers to modify an existing cube) was the "Magic Square Cube"; six sets of 3x3 magic squares in black with gold backgrounds, to be applied to the cube in a specified way. This makes the cube more difficult, as there are no colours to act as a guide, only the orientations of the digits and the fact that every line must add up to 15. This mod is of course the forerunner of the Sudokubes, although it was published several years before the Sudoku craze started.
Monoochrome QR Code stickers. Each code leads to the (English) Wikipedia page for the colour it represents.
As above but coloured for easier solving.
It is hoped that Olivér's will some day have them in stock, but in the meantime you are welcome to download these files and send them as part of a photo-cube order.
An even freakier mod is for a two-solution sticker set using stickers which change colour according to which direction you view them. They were invented by Grégoire Pfennig and are available from Olivér's. The same technology has been used to create a sticker set which, when applied to a black cube, has the stickers disappear into the background when the cube is turned.
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*World's first* Luminous Glow in dark 6 col sticker sets @ Oliverstickers.com 4 Rubik's Cubes etc
Hardest Rubik's Cube Ever Made - Now for Sale !!!
This Cube's Stickers DISAPPEAR When You Play With It !!!!