Answer:
The reason the filament heats up is because it has a high resistance, which means that as electrons move through the filament, they lose a lot of energy.
First, what is current? Current is comprised of electrons moving through an electric field from a high electric potential to a lower potential. For the current to decrease then, something would need to happen to the electrons that go into the light bulb. If 1 electron goes into the light bulb, then at the end of everything I need to still have 1 electron someplace. So how do electrons passing through the bulb make light?
Incandescent light bulbs have a small filament which when heated begins to glow and emit light. The reason the filament heats up is because it has a high resistance, which means that as electrons move through the filament, they lose a lot of energy. You can think of it as walking on a sidewalk compared to walking in waist deep water. A wire is like a sidewalk. It has some resistance, but it is so tiny that it can generally be ignored which is why wires are useful in electronic circuits. The high resistance of the light bulb is like trying to walk through waist deep water. Here energy is being taken from the electrons because of the interactions with the atoms in filament which causes those atoms to heat up, which in turn makes them emit light.
The light bulb is not doing anything to the electrons, so we expect then that any electrons going into the bulb should come out the other side. Since current is just flowing electrons, current stays the same.
Since current is the same on both sides, we know that the electrons are all moving together. Think of it like being in a big loop of people. Since everyone is in a big line you could imagine that you could only move as fast as the slowest person in the line. If everyone is on a big loop of sidewalk then everyone could run around in a circle. This is like having a large current in a loop of wire, or what we call a short. To put the equivalent of a lightbulb into our human circuit, imagine that one section of the sidewalk dips into a pool of water. Now everyone is stuck going as fast as the people trudging through the water. This is why current everywhere in a circuit is smaller when a resistor is introduced. As people trudge through the water they have to work hard to get through the water and they use energy. In a circuit, this energy comes from the voltage source, like a battery. The battery loses energy because it has to "pull" the electrons through the high resistance, and this is why the voltage drops across the light bulb
