What does T/T in advance, L/C and L/C at sight mean ?

Answer 1

T/T stands for Telegraphic Transfer
L/C stands for Letter of Credit
It's all to do with paying for items, (not too sure exactly, try posting the question in the business/finance section and you'll probably get a good response?)

Answer 2

T T is telegraphic transfer. L/C is letter of credit. At sight means on demand or on presentation for sight. These are terms used in banking, and refer to the means adopted for settling commercial dues.
A remittance made from one person to another through a bank TT involves the bank to send a (usually coded) secure telegraphic message to another branch of the bank or to its correspondent, to make the payment to the other person. In those days when electronic communication was not common, a telegram used to reach the receiver the next day and so payment could be made on the same day the telegram was received. Today, payment could be made within a matter of hours, with the development of highly efficient communication facilities.
An L/C is an undertaking by one bank given to another bank /person under which the former agrees to pay the latter an amout of bills of exchange drawn by the latter, if accompanied by certain documents of title presented by the beneficiary to the issuing bank. The L/C is given on behalf of a customer of the issuer bank in favour of a customer of the beneficiary bank for financing a commercial transaction between the two customers. The banks acts as reliable intermediaries so that the transaction could be completed smoothly.
L/C at sight is a form of drawing a bill of exchange under an L/C where the payment has to be made on presentation of the bill of exchange, subject to fulfilling the other conditions of the L/C.
The above is a simplified explanation for the terms used. You can refer to a standard banking book for the details.