Silver Tokens are the leftovers from the Gold Tokens.
Once a Gold Token has been picked up from a level, the next time the Fur Fighters revisit the same level, a Silver Token will be in its place. Like with the Gold Tokens, collecting one will restore 2HP. What they will not do, however, is count towards your Gold Token Count.
If one were to exit the level they were in (or get fluffed) but did not gather all 100 Gold Tokens, the Gold Tokens that were not yet collected will still remain and will not be replaced by the Silver Tokens.