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