source resistance
No real source of voltage is perfect. Every battery, power supply, signal generator, or amplifier output behaves as if it has a small resistor built invisibly in series with it, called its source or internal resistance. The everyday picture is a tired water pump: when no tap is open it shows full pressure, but the moment you draw a real flow, the pressure at the outlet sags, because some of the push is lost inside the pump itself.
The consequence is that a source's terminal voltage drops as you draw more current from it, by exactly the current times the internal resistance. A fresh 9 V battery with 2 ohm of internal resistance still reads about 9 V with nothing attached, but draw 1 A from it and the terminals sag to 9 - (1 times 2) = 7 V, with the missing 2 V dropped inside the battery and turned into heat. This internal resistance is precisely the Thevenin resistance of the source.
Source resistance governs how stiff or floppy a source is, how much current it can usefully deliver, and how badly it will droop under load. It is the root cause of loading and a key term in maximum power transfer. A practical tell: as a battery ages or runs down, its internal resistance climbs, which is why a nearly-dead cell can still read close to its rated voltage with a meter (which draws almost nothing) yet collapse the instant a real load asks for current.
A bench supply set to 5 V with 0.1 ohm internal resistance sags only to 5 - (2 times 0.1) = 4.8 V at 2 A, a stiff source. A coin cell with 10 ohm internal resistance would collapse to nearly nothing at the same current.
Terminal voltage equals open-circuit voltage minus current times internal resistance.
A 9 V battery is not 9 V under load; its internal resistance eats part of the voltage. A high terminal reading on a meter does not prove a battery can supply real current.