“c/o” stands for “take care of.” It is used to send a letter or package to someone at another person’s address. It is also sometimes used to send mail to a person at their place of business.
Letters sent “in care of” someone are addressed to the person they are meant for with “c/o” and the name of the person or company at the address where the letter is to be sent. The sender just writes or types a “c” followed by a “/” and then a “o” when addressing mail by hand or on a computer. On a computer, you can also make the “c/o” symbol by clicking “Insert,” then “Symbol,” and then typing the number 2105.