Send/Return
A send/return (aux send) routes a copy of an audio channel to a separate effects bus, then mixes the processed signal back into the master. This lets multiple instruments share a single reverb or delay instance, saving CPU and creating a cohesive, unified effects space across the entire mix.
In a send/return setup, you set the effect on the return channel to 100% wet (dry/wet fully wet) and control how much of each source channel is sent to it via the send knob. This is different from inserting an effect directly on a channel (insert effect), which applies the effect only to that one signal.
The primary advantage is efficiency: one high-quality reverb instance can serve all drums, synths, and vocals simultaneously, and tweaking the reverb settings changes the sound of everything at once. This is how professional mixers create the sense that all elements exist in the same acoustic space.
In FL Studio's Mixer, send/returns are configured by routing channels to a dedicated effects mixer track. Set the return track's Fruity Reverb 2 or other effect to 100% wet, then use the send knob on each source channel to control how much signal is fed to the reverb.