XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (419 page)

A rooted path represents a path starting at the root node of the tree that contains the context node.

The syntax
/A/B/C
is familiar to anyone who has used Unix filenames, though it is not actually very logical. I find it helpful to think of the
/
at the start of a rooted path expression as being a unary version of the binary
/
operator. This means that a rooted path
/X
can be considered as an abbreviation for the expression
(/)/X
, in the same way as
-3
is an abbreviation for
(0)-3
. That is, the
/