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In a neuroscience class I'm taking, it was explained that myelin covers axons in sections, the uncovered sections are called nodes of Ranvier, and signals propagate much faster in the covered sections.

But if it is faster for signals to propagate along myelinated regions of the axon, why doesn't the entire axon get covered by myelin? Wouldn't that make it faster?

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Every signal rapidly fades out with distance if it is not amplified. So if the whole axon were covered with myelin, action potentials wouldn't reach their target. Each Ranvier node can be seen as an active signal repeater.

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Action potentials arise and propagate by membrane depolarization.

A myelin sheath is present in most neurons and is responsible for insulating the membrane against ions. The function of the myelin sheath is to prevent leakage of charge, by effectively restricting ion channels only at certain regions called Nodes of Ranvier. This ensures that depolarization at one node is effectively equivalent to depolarization at another node, which initiates an action potential at the later node and as a result the action potential effectively "jumps" from node to node - this is called saltatory conduction and it speeds up the signals.

In axons without a myelin sheath, the signal does (passively) travel the full length of the axon.

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