No such thing as a stupid question!
A BBT or Basal Body Temperature thermometer is more sensitive and much more accurate than a thermometer you would get just to see if you have a fever when you are sick. Many BBTs measure to two decimal places (98.46, 98.42, 98.48, rather
than just 98.4) but it is not necessary to have a two decimal BBT (I don't and I can definitely see a pattern in my temps, but it is a BBT).
You can pick up a BBT at Target or Walgreens but I found the reviews to be all over the place on their accuracy so I ordered mine from Amazon. http://www.amazon.com/gp/product/B000GGI5UK/ref=oh_details_o04_s00_i00
HTH! =)