Meaning
It means that someone is carrying a lot of weapons, armor or equipment. Today we use the idiom "armed to the teeth" figuratively. It can be used to talk about any equipment, even abstract, not only military.
Origin
The origin of the idiom "armed to the teeth" comes from the 14th century. At that times Europe was full knights which protected areas. The phrase meant just literally "well-armored", with lots of possible weapons, with armor from head to feet.
Examples
👉 This particular PC is armed to its teeth. It’s also the only gaming PC in this particular collection that’s powered by Intel’s new Alder Lake chips.
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👉 He said the suspects were armed to the teeth with various weapons including an AK-47.