A SheetGPT token is how we measure your usage of the service. A SheetGPT token is loosely correlated with an OpenAI token which is equivalent to four characters worth of prompt or completion generation.
The general guidance is that 100 tokens of generation is equivalent to 75 average-length English words.
Keep in mind that tokens are used both in the prompt (the question or request to you make to SheetGPT) and in the completion (the response from SheetGPT). So if you make a very detailed and long request of SheetGPT, and the response is also quite long, you will use a higher number of tokens.
You can always see your current usage by looking up your license in the SheetGPT Extensions menu as detailed here.