TBD means “To Be Determined.” When setting up a choice item, you may not always know the price of your client's selection. Checking the "Show as TBD" box lets you at least show the selection information to your client, even though the pricing information is not yet available.
In the case of choices that are entered by the client, you will almost always use TBD since, until you see the exact choice of your client, you cannot know what the price will be.
Even if a selection choice is set to "Show as TBD", you can still enter in tentative pricing information or, for projects where you're tracking the budget, build your costs for those lines.
As long as the choice is showing as "TBD", your clients will not see any of the tentative pricing.
But, make sure you come back in and update the option when the pricing is all finalized so that you can show the accurate pricing to your clients and track the accurate costs in your budget.