Hi and welcome. You've picked the perfect place to post.
When you turn after casting on 6sts and purl 12sts, you'll be purling back over the 6 cast on and 6 of the original 12sts (bolded) from the beginning of the row. That'll leave 6sts sitting unworked. The 12sts you're working on now for 10rows will actually form the thumb. You'll get back to the 6sts that are hanging out on the needle after the thumb is finished.
"Next Row (RS): K12, turn, cast on 6 sts, p12, turn.
Cont. to work on these 12 sts only, leaving rem 6 sts on needle.
Cont. to work in st st for a further 10 rows."
Can you post a link to the pattern?