In simple words: It means the engine is an integral part of the frame.
Advantage: Weight reduction and if done correctly improvement in handling too!
Now in geeky detail: In a automobile (both the 4 wheeled type and the 2 wheel type) when the engine is acting as a stressed member, the suspension loads are fed first into the gearbox and then through the engine into the chassis. In this type of engine design the block itself has to be stiff enough to cope with these suspension loads being fed into it via the gearbox end and it into the chassis at the other, otherwise bore distortion can occur. I have also read that production blocks in cars are rarely designed to be suitable for use as a stressed member but this technology is fairly common among various sports oriented designs in cars, like few Ferraris (1995 F50 was first) and F1 cars and LeMans racers. But this type of setup is fairly common in high performance bikes (like busa) and was first featured in a sunbeam in 60's (I guess
). Previously in India LML bikes used to feature this type of engines and now it has been featured again in Pulsar 135 LS.