the physical layer
Imagine you and a friend in different rooms, joined by a piece of string. You agree to tug the string twice for yes and once for no. The string is the path, and the tugs are the signals you actually send. The physical layer is exactly this lowest, most concrete part of a network: the wire, the fiber, or the open air, plus the rules for turning a bit (a 0 or a 1) into a real physical thing that can travel across it.
Everything above it deals in bits as abstract symbols. The physical layer is where those symbols become voltage levels on copper, pulses of light in glass, or radio waves in the air. Its job is narrow but vital: take a stream of 0s and 1s from the layer above (the data link layer), encode each into a signal pattern the medium can carry, push it out, and on the far side measure the incoming signal and decide whether each instant was a 0 or a 1. It does not understand addresses, messages, or which application sent the data — only signals and bits.
Because it is the foundation, its limits become everyone else's limits. The medium and the way you signal on it set how fast you can send bits (the data rate), how far the signal survives before fading (attenuation), and how often a 0 arrives looking like a 1 (the bit error rate). A flaky physical layer makes even the cleverest higher-layer protocol struggle, which is why the rest of the stack is built to cope with the imperfect signals this layer hands up.
When you plug an Ethernet cable into a laptop, the physical layer is what turns the byte 01001000 into eight precisely-timed voltage changes on the copper pairs — and on the other end, samples those voltages back into 01001000.
The physical layer carries bits as signals; it neither knows nor cares what those bits mean.
In the OSI model the physical layer is layer 1 of seven; the real Internet's TCP/IP model just calls everything below IP the 'link' and folds the physical details into it. The split is a teaching tool, and in practice the boundary between physical and data-link blurs.