A chaser tool looks like this:
Its almost like a tap. You just insert it in first and it cleans the threads (apply some vasaline to gather the dirt). Sometimes you can get dirt and all kinds of crap in those threads.
I never did this. I just blew some compressed air on the plugs before removing to install the new ones.
The lima block has the plugs at an angle so there isn't that big of a pocket for dirt to fall into. My mustang has completely vertical plug holes and they are like 6 inches deep, more likely to trap dirt.