As a shareholder in a pass-through entity you need to be tracking your tax basis in that investment. That is what TT is asking for.
Your tax basis begins with your initial capital contribution and is updated annually for the applicable lines on the K-1.
Don't know how long you have had this investment, but you will need to determine this. This can be done with a simple excel worksheet that will track it by year and then cumulative.
See the attached discussion. While the date indicates 2015, you can still use this as guidance. Just update any line numbers, however, doubtful much has changed.
https://www.irs.gov/businesses/small-businesses-self-employed/s-corporation-stock-and-debt-basis
If you have any questions on this the individual that prepares the 1120S return should be able to provide some guidance as well.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.