To put it bluntly, downloading routes in gpx format means that you will always have your position on your mobile phone, even when there is no phone signal, on a detailed map of the trails with a highlighted route to follow... a great convenience, especially for those who, like me, have a certain tendency to get lost. To do this, you need to download a programme or application that reads this format and allows you to download the maps and routes you are interested in for offline use.
I use locus map as an application on Android and I get on well, but I am sure you will find someone more competent than me on the net to advise you.