Enclosed means it is included with the letter. Literally, it is "closed-in" the envelope. However, I also use this for describing email attachments at work. "Please find enclosed...."

It is used at the end of the letter, examples are as follows: 1, A check is enclosed herewith. 随信附寄支票一张。 2,"Enclosed you will find the account of the premium, for which I have debited your account &40.00." 同函奉上保险费帐目一份,上开金额为40英镑,我已记入贵方借方帐户。 3,Enclosed please find the invoice of eighty bales cotton bought by your order, and shipped to Kobe per" Inaba Maru", as per the enclosed B/ L. 根据贵公司的定单,已将80包棉花装于驶往神户的"因幡号"货轮。同函奉寄发票及提单各一份,请查收。 hope it helps^^