Bit rot is the gradual corruption of stored data — individual bits flipping from 0 to 1 or 1 to 0 due to physical degradation, cosmic radiation, or simply the passage of time. A single flipped bit in a text file might change one character. A single flipped bit in an executable might make it crash. A single flipped bit in an encrypted file might make the entire file unrecoverable.
The rate of bit rot depends on the storage medium and the environment. Hard drives have an uncorrectable error rate of approximately one bit per 10^14 bits read — roughly one error per 12.5 terabytes. That sounds negligible, but a modern data centre reads petabytes per day. Cosmic radiation — high-energy particles from space — can flip bits in RAM and storage. At sea level, a computer with 16 GB of RAM experiences approximately one bit flip per week from cosmic rays.
File systems and storage protocols include error detection and correction. RAID arrays can reconstruct data from parity information. The ZFS file system checksums every block and can repair corruption from redundant copies. But these protections are not universal. Consumer devices — laptops, phones, USB drives — typically have no protection against bit rot beyond the storage medium's built-in error correction, which is limited.
Bit rot is dissolution at the smallest possible scale — one bit at a time. It is invisible until it is catastrophic. A photograph that has accumulated bit rot may display a single pixel of the wrong colour; or it may not open at all. The dissolution is stochastic. You cannot predict which bits will rot or when. You can only make copies and hope.