You can use marks to save locations, and to record and access information about a location, and to navigate to a location. You can also use marks to measure the distance and bearing to a point.
To create a Mark, touch the Mark button 
on the menu/toolbar.
A mark is created at the center of the screen, and you can drag it to
adjust its position with the green four-way arrow handle. If you touch
anywhere else on the map, the handle disappears, but it is shown again
if you tap on the location of the mark. (Note, you have to select the
point at the bottom of the flag-staff, not the top of the flag).
To record your current GPS position, first touch the lock button 
to scroll
the map so the GPS is at the center of the screen, then touch the Mark
button to create a mark. If your device has an app launch button (such
as a dedicated camera button), pressing and holding this button creates
a mark at the current GPS location.
To measure a bearing and distance from your current location, create a mark and drag it to the point you are interested in. The bearing and distance is displayed above the mark.
A blue pop-up menu button is shown when the mark is selected. Touching this allows you to delete the mark, edit it, or, if it has an associated URL, open information about the mark in the web browser. For example, if you import a GPX file containing points of interest, you can access detailed information on the web about each point. Editing a mark allows you to give it a meaningful name, change the icon or color, edit the position using lat-long or grid coordinates, enter a text comment, or enter the linked URL.
If you select "Go To" in the popup menu, you can use the data panel arrow pointer and navigation data to help you get to that location. The data panel can be customized to give you more detailed information such as ETA, VMG, etc. Turn the device to landscape mode to show even more data.
See below for information on importing and exporting marks.