In English you call this indefinite article. In German "unbestimmter Artikel".
It is very similar to "a" and "an" in English.
You use it when something is not defined.
Ein Baum an der Straße = A tree near the road
--> A random tree. Not specified.
In contrast you have normal articles like "die", "der" und "das".
You use them when it is rather clear what you talk about.
Der Baum unter dem wir uns geküsst haben = The tree where we have kissed.
In this case you know that they kissed only under one tree.
In contrast: Ein Baum unter dem wir uns geküsst haben = A tree where we kissed.
About the sentence:
First you say that it is one of the last idyls and then the ":" signals that a list beginns. And the list contains the positive aspects of the place.