While in standard English a mate is a life partner, it is commonly used in Britain to mean a friend. It is also often used to address strangers in informal situations, such as in bars or on public transport. It is particularly used between men (but not always). A similar word is “pal” (which is also used in American English).
“What are you doing this weekend?”
“Hanging out with some of my mates.”
“Excuse me, mate, is anyone sitting here?”
“Hey pal, could I get a whisky and a beer please?”