Mr Tate was trying to dig a hole in the floor with the toe of his boot...

In this metaphor, the floor represents the whole scheme of things, the whole problem with Boo Radley and how he actually killed Bob Ewell but Atticus and Heck Tate don’t want to bring him into the limelight. The metaphor represents how by saying that Bob Ewell accidentally killed himself, Mr Tate is desperately trying to find a way that Boo Radley doesn’t get brought into the limelight, digging a hole in the big problem with the toe of his boot.   The physical action of him digging with his boot shows how he is nervous about what he is doing, he doesnt want to speak directly about what he is going to do.