What is the significance of cysteine in a protein sequence?
What can I infer if I get a high percentage of C from a protein
  sequence?


A highly stable structure that is likely found in the extra-cellular space.



Cysteine can form a disulphide bond with another cysteine. Cysteine can be found as a lone cysteine, but is often paired with another cysteine in the tertiary structure to form these bonds.

Disulphide bonds play important roles in protein folding and stability (60 kcal/mol compared to around 1 and 5 kcal/mol for a hydrogen bond depeneding on the environment). Notably though, cysteine disulphide bonds are usually only used in extracellular secreted proteins, as they are unstable in the cytoplasm.

As an example, take the structure 2ksk. Look at how the structure is held together by cysteines that are distant in the sequence. If you see cysteines in a sequence, expect interesting folding!

The cartoon is going from blue to red, whilst cysteines are shown with sticks and the S-S bonds are in yellow.



Side note, they are annotated as SSBOND in the PDB.