A Radler (Shandy) is a light refreshing beer-based drink that combines beer with a citrus-flavoured soda. The term "Radler" originates in Germany and means "Cyclist". The drink was originally designed to soothe thirsty cyclists with a refreshing lower alcohol option.
In the UK it's basically what we would call a Shandy, a sweet low-alcohol option with all the crisp carbonation of a beer, perfect for when you need to stay level-headed.