The following examples are individually commented.

/*                         root element without knowing the name

//title                    title anywhere below context element

memo/title                 title under memo in context element
memo//title                any title anywhere in memo
//memo/title               title in any memo
//memo//title              title anywhere in any memo in document

child::memo/title[attribute::priority='hot']      long notation
memo/title[@priority='hot']                       identical in shorthand
memo/title[@priority]                             any title with priority attribute under memo in context
memo/title[priority='hot']                        any title with priority under memo in context
//title[@priority='hot']/date                     date child in hot title

memo/title[first()]                               first title under memo in context
memo/title[1]                                     same as above
memo/title[position(1)]                           same again
memo/title[last()]                                last title ...
memo/title[last()-1]                              penultimate