Firstly it helps to understand what a zip file is. It is a bit like container for other files, bit like a suitcase with a zip to keep things in. Like a suitcase it can squish things into less space than they would take up in a pile just on the floor.
Also like a suitcase – you can’t use your t-shirt unless you unpack it first.
Luckily unzipping/extracting/opening zip files is pretty simple.
On windows – a right click and select extract all – will unpack it all, similarly on a mac – but a double click.
On a phone – well this is a different case – so many phone types with different versions of software. Maybe it is built in – maybe it isn’t. There are plenty of apps to do it. For Android I like file viewer – it also lets you view SVG files which can be very convenient.
For ipad and other i things – you could try the free version of winzip
or on any device use an online unzipper such as http://unzip-online.com/ which you just upload the zip then download the extracted files.
Why do people go zipping up all these files?
Few reasons – keeps them all together for easier sending, makes them a bit smaller – for quicker upload or to avoid file type restrictions such as facebook not allowing svg files.