This window lets you choose the Parts you want to join by checking boxes in a matrix.
If you have a large number of Parts, you may find it easy to join / unjoin Parts using the Join / Unjoin options in the Joins dialog panel.
The matrix shown in this panel list the Parts of your model vertically and horizontally. This matrix is symmetrical, so only the diagonal and lower part of the matrix is shown.
Check a box to join the two associated Parts.
The Remove buttons let you quickly uncheck the boxes for each row of the matrix.
Press to apply the changes you make to the matrix.