That's a generic term in OO. It refers to objects whose instances are uncomplicated and small. That is typically done by keeping the real data in some kind of container which does the management. A blessed scalar key to the class global container is then a lightweight object.

It's sometimes a bit of a cheat - the container and its management methods are likely just as complicated as a direct implementation would be.

There's a lot of information here, just browse the list in this link to flyweights.