In the context of family law, the word would typically refer to the person obliged to make child support or maintenance payment to the payee or recipient pursuant to a Court order or a separation or other child support agreement.
For an example of usage in a statute, Nova Scotia's Maintenance Enforcement Act (1994-95 SNS Chapter 6 published at canlii.org///ns/laws/sta/1994-95c.6/index.html) defines a payor as:
"... a person required under a maintenance order to pay maintenance."
Or Prince Edward Island's Maintenance Enforcement Act (1988 RSPEI Chapter M-1):
"'payor' means a person who is required to pay maintenance or support pursuant to a maintenance order."
In commercial law, the word refers to the person who makes the payment on a cheque or check or bill of exchange.