Learn Icelandic: Sætleikurinn í safanum skiptir miklu máli. - The sweetness in the juice matters a lot.

It is derived from the adjective sætur (sweet). By attaching the suffix -leikur, it becomes the strong masculine abstract noun sætleikur (sweetness). The -inn at the end is simply the attached definite article.

Is there a difference between sætleikur and sætleiki?

No, they mean the exact same thing. Many abstract nouns derived from adjectives have two forms: one ending in the strong masculine suffix -leikur and another in the weak masculine suffix -leiki. You will hear both sætleikur and sætleiki used interchangeably by native speakers.

Why are miklu and máli in the dative case?

They are governed by the verb skipta. The phrase skipta máli is a fixed expression meaning "to matter" or "to make a difference," and skipta always takes a dative object in this context. Here, the adjective mikill (much/a lot) is added to modify mál, so both words take the neuter dative singular form to become miklu máli.

Why does the preposition í trigger the dative case for safanum here instead of the accusative?

The preposition í governs the dative case when it describes a static location, meaning where something currently is. Because the sweetness is already residing inside the juice, it takes the dative. If the sentence described an action of movement or direction getting put into the juice, í would take the accusative.