A full lug barrel has the metal frame (or tube) that the ejection rod sits inside of extended all the way down the barrel to the end of the muzzle.
It's located underneath the barrel and makes that barrel heavier and consequently easier to point and aim. Here, a picture is worth a thousand words -
By the way, the S&W Model 17 (K frame, 'Combat Masterpiece') is one of my favorite pistols. I've had mine for more than 35 years; and, it's going to be one of the very last of my pistols to go. You made an excellent choice - One that you'll always be able to afford to shoot and practice with.