Translation:The two countries have an agreement.
"överenskommelse" appears to be a compound word. Could you please explain the components? If any. It might make it easier to remember it.
The ultimate origin is en, which of course means "one", and ense was an adjective form meaning to be one or the same.
över, as you probably know, means "over", and has a fairly wide usage as a prefix - much like e.g. "over-" in "overcome" isn't strictly relating to the word's topological sense.
So överens, then, is to be in agreement. The idiomatic phrase is att komma överens meaning to come into agreement. I imagine since English uses a noun (agreement) and Swedish an adjective (överens), the phrasing differs slightly between the languages.
Hence, with -else being a common suffix that creates nouns (like e.g. "-ness" in English), an överenskommelse is literally an "over-agreed-come-ing", so to speak. I hope that helps.