It means an example of something that includes the claimed invention. An embodiment is a concrete thing or process that practices the more abstract “invention-as-claimed”. Implementation is close to the meaning of embodiment.
The invention itself is defined by one or more numbered claims. Claims are written in a way to apply to more than a single embodiment.
If a claim says “a device for electrical connection comprising two sets of physically complimentary electrical contacts in housings, the housings operatively retained together magnetically without requiring mutual friction to maintain a physically secure electrical connection”, then an embodiment would be Apple’s MagSafe power connectors for its laptops. An alternative embodiment might be housings retained electromagnetically rather than by permanent magnets or be large enough to operate as part of a golf cart charging system rather than a laptop.
This example is for a device. For software it would be analogous with a general claim and usually one or more specific embodiments. Very roughly speaking, a claim would define a general principle and an embodiment would be a specific implementation using that principle.