Complete NEWB and openpyxl project

[Netopia]

OK, so I did some reading and I think I understand the block below, but need some clarification:

#print each group in desired order
for group, rows in sorted(groups.items()):
    print('\n\nGroup {}\n---------------\n'.format(group))
    for row in sorted(rows, key=lambda x: x[-1][:-1]):
        print(row)

I read some on 'lambda', and I get the key defined as X and x being the last element in rod, but I don't get the usages of the [:-1]

Joe