What's the difference between the "main" and "lead" roles in KPop?

The term 'lead' in 'lead vocal' in KPOP actually means 'guide'. The main parts (or let us say, the heavy/difficult parts and chorus) usually belong to main vocalists, while the lead vocalists often take the parts where are pre-chorus, verses, not-that-difficult-part of main parts/chorus, or the beginning of the song (lead other vocalists). For example, in WJSN, lead vocalist Seola sings the first sentence of lyrics in their songs (e.g. Cantabile, I-YAH).

But this could be different varying with different groups, or according to different songs, which means lead vocalist could also sing the main/difficult/chorus parts of a song (very common, such like TWICE Nayeon (Nayeon is usually distributed to chorus parts together with their main vocalist Jihyo) and Jeongyeon (e.g. check their new song CRY FOR ME)), contrasted with Jihyo, to whom the chorus always distributed.

This 'lead vocalist sings main parts/chorus' also happens especially when there are not many members in a group, or only one main vocalist in a group, where the line distributions would be flexible. For instance, in BlackPink Rosé and Jisoo always share the main vocal parts together. There is an interesting example. In Kill This Love, both Rosé and Jisoo sing the pre-chorus parts (the choruses of BP's title songs are often nearly vocal-blank, which is why pre-chorus happens here). But you could still hear the first turn of second part of pre-chorus (from 나 어떡해 나약한 날 견딜 수 없어) belongs to Rosé, which I believe there are some reasons why they adopt such turns.

In effect and personally, I would like to say that there are not many differences between lead and main vocal except high notes' parts.

Additionally, the Korean noun of lead vocal is 리드보컬 (abbrev. 리보) and main vocal is 메인보컬 (abbrev. 메보). Both of them are transliterated words loaned from English.

Check the terms' explanation here and the differentiation here.

The main vocalist usually receives the majority of vocal lines per song, especially the most challenging ones, because of their broad and steady vocal range. They often support the lead vocalist during the chorus, and are also more likely to receive solo parts than sub vocalists are.

The lead vocalist is actually secondary to the main vocalist, but takes the vocal lead in songs by singing before the main vocalist(s) during verses.

Read this. The explanation is clear