Retirement tax questions

ahhh......so since I have never taken out of my Roth IRA before, Ive never been required to file 8606.

 

The only reason I got an 8606 was because I converted my traditional IRA to a Roth in 1998.  I paid the taxes out over 4 years.   I just want to understand what basis is.  On my worksheet, Turbo Tax has been recording the same number each year......in my case $40,127.  Its on the line that says "Cumulative pre xxx year conversions-taxable and non taxable".  im guessing that this $40,127 is my basis?  Therefore since I withdrew $40,000, none of it would be taxable?