Need help with Tax code! What is this?

0T W1 is an emergency code because this is a second job, and it's being assumed that all your £11,000 personal allowance is being used against the other job. (Or you told Iceland it isn't your only job.) The 0 means you have no tax allowance, so everything will be taxed at 20%. W1 means Week 1 basis, that is to say, it is a non-cumulative code - don't worry about what that means.

If you are earning less than the allowance in both jobs, you need your allowance split between the two. HMRC will notice and adjust this eventually if you're still getting paid from both. But ARE you still being paid by both? If not, contact HMRC and get them to review the code https://www.gov.uk/government/organisations/hm-revenue-customs/contact/income-tax-enquiries-for-individuals-pensioners-and-employees

This code assumes that all of Your yearly tax allowance - 11K is used up.iceland should have given you a starters form to fill in.
You need to phone HMRC.
I Presume you have stopped work for the Agency, if so they should have given you a P45 when you left.
You will get a refund if you have overpaid.

if ths is your only employment then you are on the wrong code. The agency has kept you on the books and the system has messed up (are we surprised?).
Go to gov.uk/income tax - either follow the instructions or get the HMRC phone number and call in.
Once you get an answer the code will be corrected overnight and any excess income tax will be refunded through your wages.

0T Wk1 means no allowances so paying at least 20% income tax.

If working for the shop AND the agency make an estimate of your earnings with each employer before you call in - the call handler will advise.