"Plug" is an interesting word here. In this context I think it means "to input data values into a database."
1) It comes originally from the idea of electrical or audio connections. The "plug" is at the end of the device's cable. You put the plug into a "socket" or "jack." This is called "plugging in" the device.
Plugs and sockets are standardized. You can choose which of several devices to plug in. You can plug in a lamp, or a toaster, or a vacuum cleaner.
2) In algebra, letters like "x" stand for variables. The variables can have any value. When you are evaluating an algebraic expression, you can choose any value you like. In formal language, this is called "substituting" a value for the variable. Thus, in the expression 5x + 1, we can substitute 10 for x, and the value of the expression is then 51.
3) In informal language, substituting a value for a variable is often called "plugging in a value." In the expression 5x + 1, "if we plug in 10, we get 51. If we plug in 100, we get 501."
4) Some computer languages, including database programming languages like SQL, also contain the idea of a "variable" to which a value can be "assigned." This, too, can be referred to as "plugging in a value."