A Payment is the means by which amounts are paid to your Suppliers: it represents a physical financial transfer of an amount of money.
A Payment can be a Supplier Payment, created for a specific Supplier (and therefore allocating a single amount to their account) and allocated to one or more Supplier invoices for that Supplier. Only Supplier invoices for the specified Supplier can be allocated an amount.
A Payment can also be a Manual Payment, distributing amounts to one or more allocation accounts, such as Expense or Cost of Sales accounts.