Fusion is a standard biological component of the Gems. Whether they plan it out in advance or achieve it spontaneously, Fusion can be activated when two or more Gems are in sync. When melded, the Gems' minds become one and the resulting Fusion Gem carries their memories and traits while simultaneously being her own Gem. They're considered an individual with their own personality and sense of self, though they are fully aware that they are a product of multiple Gems.
Fusion isn't as simple as transforming into a bigger Gem, but a huge part of their culture. How Fusion is viewed and activated varies amongst the numerous Gem species and as is often wont of a civilization, it's much more complicated than it looks. Some do it for the power it brings while others bond through friendship or romantic love. Fusion can be willfully permanent if the non-fused Gems choose it to be or the Fusion herself may overwhelm her non-fused counterparts and take over. Fusions isn't a perfect algorithm: they can materalize based on the strongest parts of the non-fused Gems' personalities, but they don't necessarily need to be positively-defined traits. Non-fused Gems can also differ in opinion during a Fusion, causing the latter to split apart. Ideally, Fusion is at its best when the non-fused Gems equally consent to the act and maintain a harmonious sync.
In Homeworld, Fusion is divided by its rigid social structure. Gems who fuse are relegated to their own class and type; anyone who attempts to fuse above their station is punishable by death. There are Gems who feel differently and often keep their controversial transformation away from prying eyes. This is especially notable for the Gems living on Earth where they have the freedom to express their feelings and will through Fusion. Homeworld has even attempted to weaponize Fusion, using shattered Gems from the war to forcefully mash them, turning them into monstrous creatures with no will to speak of. These prototypes were gateways to the Cluster, a gigantic mega-fusion comprised of millions of Gem Shards and buried deep inside the Earth. When it hatches, it will take out the entire world.
Fusion's impact on Gem society is monumental. How each Gem fuses depend on their uprising and attitude, so no two Gems will share the same view. That's what makes Fusion so great, it's a chance to express your diversity and feelings into a physical act. Homeworld may not get that, but on Earth, even a half-Gem like Steven can fuse and live vicariously through it.