The Avalanche Effect refers to the fact that for a good cipher, changes in the plaintext affect the ciphertext. The algorithm produces a completely different output for a minimally changed input. For example, the SHA-2 checksum algorithm or the AES encryption algorithm show a strong avalanche effect. If you change only one character in the input (for example, instead of aaaaaaaaaaaa now aaaaaaaaaaab), the outputs are still significantly different (for example, c1e768d3 9177e9ef debee33a b92b4450 instead of 0a561d9e 30bb09db 47f8e83d 443865cf). Encoding by monoalphabetic substitution, on the other hand, shows no avalanche effect (example: input 1: aaaaaaaaaaa and output 1: rrrrrrrrrrrrrrr; input 2: aaaaaaaaaab and output 2: rrrrrrrrrrrrz).
The Strict Avalanche Criterion means that half of the output bits change when an input bit is changed.