Diffuser und Bouncer
Under these names you can get a variety of accessories with the same objective: To make the flash light softer and more natural.
A diffuser scatters the light to make it diffuse.
Provided that it will then be reflected from the ceiling, a wall or the floor towards your subject additional light coming from the side will make the light softer.
On the left you see an illustration of this principle and a real sample.
Note that this effect works only across small distances of a few meters and that you will only lighten the enviroment without improving your pictures if there is nothing to reflect the scattered light.
You can buy diffusers cheaply but every self made solution where you place some transparent, cloudy material over the flash works too.
Do not expect drastic improvements, also the effect varies with the distance and the reflections. I have usually seen only very minor changes from a diffuser.
Sometimes these terms are mixed, possibly you find something labeled „bouncer“ looking like what is described here as a „diffuser“. However I find the distinction makes sense, it is consistent with the terms and the two types of accessories work differently.
Bouncers are also available as accessories or can be created with simple means instead. A piece of white cardboard, fixed with tape or an elastic band does the same trick. The bigger the bouncer is, the better.
The effect of bouncers also varies with your shooting situation, with small ones like the one above I have noticed only minor differences.
These two variants are simple accessories that are easy to carry.
Better effects are created by bigger attachments, so called „softboxes“.