A hedge fund is a way to pool money from advanced/experienced investors (i.e. institutional investors such as pension companies, or high net worth individuals etc - not Joe Bloggs with £500) to invest in companies according to a specific "strategy" which they hope will give them consistently high returns, higher than just sticking it in a bank account. Usually but not always these days, the goal is also to "hedge" against risk, so your strategy is set up so that overall you don't lose money even when the market is down.
The strategies can be very technical, but it just refers to which companies you buy and sell at what times and in response to what events. You can also bet against stocks, so that you make money if they fall in price rather than rise. You can take in automated data feeds and have algorithms do your trades for you, or you could do boots on the ground research to give yourself an edge, anything you like really.
A hedge fund manager will have targets on how much risk to take and desired return. He (pretty much always he) spends his days talking with his team about how to design the strategy/algorithm, discussing changes to make it more accurate, looking at reports from the last few years about the fund performance, talking personally to investors, etc. Probably not actually clicking buttons or getting on the phone to make the individual trades, as that would be more junior, but people in the office would be doing that.
They start the day early, before the markets open so they can discuss their strategy for the day. The vibe is somewhere between investment banking and a tech startup - intense, quite collaborative but direct and challenging, lots of data, looking at screens, that kind of thing.