Mike, I've just had a quick peek at my (Android) app and there is no legend there. You might be better to look at (and play around with) the map page on the website first to get familiar with the process. The legend can be accessed in the top right of the screen (website) by tapping on 'key'.

The yellow circles I believe you are referring to are, indeed, km (or mile) markers. (We can choose which in our 'Profile').

It's probably worth remembering that as great as the app is, the website is superior for planning with more features on offer. The app is particularly well suited for following planned routes and creating quick, simple ones. Once a route is saved on the website, it's available on the app.

Even if I only have my phone, I'll plan a route on the website on the phone.