A slippery sequence is a small section of codon nucleotide sequences (usually UUUAAAC) that controls the rate of ribosomal frameshifting. A slippery sequence causes a faster ribosomal transfer which in turn can cause the reading ribosome to "slip." This allows a tRNA to shift by 1 base after it has paired with its anticodon, changing the reading frame.
References
Slippery sequence Wikipedia(Text) CC BY-SA