To use the save system first you need the SaveSystem actor in your level, you can find this in the content browser in the Blueprints folder. You can drag this in to your level to add it.
Next you can select the SaveSystem actor thats now in your level and in the details panel you will see a few options.
By default the save system will be set to auto save every 20 seconds and when the game start will try to load it previous save, if there is no save it will just start a new game.
You can disable auto save by unticking the Auto Save option.
You can also disable the save loading by unticking the Load Save option. This means when the game start the save system wont try to load a previous save.
The Save Name is the name the system will use for its auto saves. Its also the name of the save the system will use when it try to load a save game at the start of playing.
Auto Save Timer is the number of second between auto saves in seconds so the default time is 20 seconds. You can change this to be what ever you like.
There are also debug setting that will display text when a save is done and when a load save is done to make it easy to see when those actions are happening.
Last there is the delete save button, this is very useful for when you are building you game in the editor as it allows you to delete saves quickly without having to go in to the project files.