https://en.m.wikipedia.org/wiki/Key_whitening
Presumably something like rot13 would count as whitening? Also, assuming the name comes from analogy with "white noise", ie reducing signal quickly, cheaply?
Say you encrypt a message M with key K1 and get encrypted message E: encrypt(M, K1) = E. An attacker might brute force your encryption, if your key space is small this might be an issue. So what you can do is XOR the message with another key K2 before encryption and a third key K3 after encryption to get: E = encrypt(M XOR K2, K1) XOR K3. Now the attacker has three keys to brute force. (though I think the actual effective key size is between 2x and 3x the length if the attacker knows the message distribution)
I'm not an expert but I imagine XOR is popular because it's a basic logical operator and so has gate level hardware implementations.