Loading a map
If you look at the bottom left of your screen you should see a ribbon that says "Map Selection". If you click on it, a window will expand with all the maps in their respective categories (eg: Bronze Sea, Nimbus Sea). You can click on the individual map names to load the respective map.
Map Interactions
Now that you have a map loaded, here's some things you can interact with in the map:
You may see that either on the top right or the top left there's a legend, it displays the icon of each marker and a short phrase describing it, you can click the icon of each of those markers to toggle the marker layer entirely.
Some maps will have more than one image associated with them, if thats the case, you'll be able to see either on the left or the right margin an arrow-like button that you can click to scroll through the different images.
If you hover over a marker, it will tell you the marker legend (if you hadn't already read it), and if it has any extra text associated to it (a note), it will also display it.
About zoom
If you look at the bottom right, you'll see a "+" button, a "-" button, and a small display with a percentage. These refer to the zoom level, you can zoom in or out 10% at a time by clicking either the "+" or "-" buttons. When you are zoomed in, you'll get scroll bars for either the horizontal or vertical direction, letting you scroll through the entire map.
About the numbers
Some markers will have numbers written on top of them, if it's a chest or a private chest marker, it means quantity, if it's a cave entrance marker or similar, it usually means a link between that marker and another marker of the same number.