What does underlined mean in man pages
 

I have tried reading many things and was never able to clear this up much. I tried man man and as much as I was able to digest of that, I couldn't figure out the answer to the following. What does it mean when a man page underlines something. For example here is a snipit of output from man date.
Obviously I wanna know more about the format STRING should be in. Is that why its underderlined? Where/how do I get this info? I am actually looking for what I can pass the date parameter in the date command, so if the underline isnt the actual answer, help with that would also be appretiated.

Interesting question. It seems to me at first glance that everything that is underlined is something you'd replace with something else rather than typing verbatim. For example you have to type "--date" not "--iwantthisinstead" but you don't literally type "STRING" or "FILE" (you replace it with whatever you need to).

I played around with the command for a minute and it looks like -d allows you to print date information for another time, like "date -d yesterday" and "date -d tomorrow".