Solo Challenge Tokens are only offered to players who purchase Rewards Pass Premium. You can use this to skip timers for replaying or advancing to the next WordMaster.

The initial purchase will give you tokens. You can get a max of 7 tokens, and the start of every new, subsequent Solo Challenge Week will give you tokens as well.

After exhausting all the tokens you have, you can then Watch an Ad and/or spend coins, to replay or advance to the next WordMaster. 

Check out the GIF below for a demonstration on how to earn tokens.