'u' is the displacement degree of freedom about global X-axis direction.
'v' is the displacement degree of freedom about global Y-axis direction.
'w' is the displacement degree of freedom about global Z-axis direction.
'x' is the rotational degree of freedom about global X-axis direction.
'y' is the rotational degree of freedom about global Y-axis direction.
'z' is the rotational degree of freedom about global Z-axis direction.
Checked checkbox represents restrain against the degree of freedom.
Normally you keep all the checkboxes checked for a fixed support.