American English is a set of dialects of the English language used mostly in the United States (Wikipedia).
The grammar differences between British and American English are minor and NORMALLY do not affect mutual intelligibility. However, their differences in phonology, phonetics and vocabulary are a source of amusement on both sides of the Atlantic.
In the UK we note that British English is becoming increasingly "Americanised". Naturally, estate agents find it easier to sell fancy apartments rather than boring old flats, and cinemas advertise the latest movie showing - now that's entertainment, not like those films we used to watch.
To hold the fort. To look after something or assume someone's responsibilities while they are away.