Help understanding solution

Solved it and tried to understand solutions of other people. Can't figure out >items[indexes[x]:indexes[x+1]][::-1]< in the following solution:

def reverse_ascending(items):
indexes = [0, ]
for x in range(len(items)-1):
    if items[x] >= items[x+1]:
        indexes.append(x+1)
indexes.append(len(items))
new_items = []
for x in range(len(indexes)-1):
    new_items.extend(items[indexes[x]:indexes[x+1]][::-1])
return new_items

I undestand that array[x][y] is used if array[x] is itself an array. I don't see how in the above >indexes< gets a nested array inside and how this indexing works. Couldn't find on google (maybe coz I don't know how to search). Thx in advance for any explanation.