A reverb is the sound of the surroundings or the environment. Different buildings will have a different sounding reverb to them.
The Convolver plugin comes with quite a bit of reverb effect samples, called impulses.
To use the plugin, select a track in the Mixer window.
Add Convolver plugin into one of the FX slots for that track.
In the File Browser window of FL Studio, look for Impulses folder.
Choose a sample you'd like to try out on your sound, then drag and drop it into the Convolver plugin, onto the area where you see the sound wave.
The plugin has some parameter knobs to tweak that reverb effect.
Dry, Wet, and Delay knobs accomplish what they usually accomplish in other plugins. "Dry" sound in music software refers to sound before effects are applied. "Wet" sound refers to the sound after the effects have been applied. Delay, well, applies a delay to the effect.
Self-conv knob is essentially a double reverb, it applies a reverb to a reverb effect.
Stretch adjusts the length of the impulse response. You can make the response super long, or very short.
Eq knob adjusts the wet and dry amount.
Below the knobs, in the top right, is a toggle for equalizer.
If you toggle the equalizer view, you will get a frequency graph, with frequencies on the x-axis, and time on the y-axis. So it essentially allows you to alter the frequency of the impulse over time. You can draw a custom frequency curve by adjusting points of the line.
You could find a lot of reverb impulses on the internet for free, from people who have recorded different live environments; and use them to give your sound a more realistic feel, with the help of the Convolver plugin.