Hello! First, I will address what a neuron is. Quite simply, a neuron is a cell within the nervous system. Its job is to carry electrochemical messages from the brain to other cells.
Components of a neuron.
Basically a neuron is made up of a cell body, axons, and dendrites. Dendrites bring electrical signals to the cell body itself, while axons bring electrical signals away from the cell body. The cell body of a neuron contains all the same types of organelles as other cells in our body, namely the nucleus, cytoplasm, endoplasmic reticulum, mitochodria, and nucleolus. When a neuron transmits information, a dendrite will receive an electrical signal from another dendrite (belonging to another neuron) and pass the information along the axon. The point of contact between dendrites is a synapse. The axon is covered with a layered myelin sheath made from glia cells- this accelerates the transmission of information or electrical signals along the axon.
How it works
When a neuron is not sending any signals whatsoever, we say it is at rest. This is what we call the resting potential. An action potential occurs when a) a neuron sends electrical signals down the axon b) when depolarization occurs at about -55mV. What is depolarization? This is electrical activity caused by a depolarizing current. Most internal cells are negatively charged; when depolarization occurs, the cell becomes positively charged for a brief period, allowing for the transmission of electrical signals.
When depolarization occurs at about -55mV (this is called the threshold level), an action potential will fire. This just means an electrical signal is being sent down the axon. If the threshold level is not reached, there simply will not be an action potential. This is the all-or-none principle that you asked about.
I hope this is helpful. For more details, please refer to the sources below. Thanks for the question!