Lighting modifiers are a broad category including reflectors, diffusers, gels, physical modifiers of light like barn doors and cones or snoots to direct and focus light. This is not a complete list, but more of a start into some of the basic tools which can help with photographing bonsai.
You can use lighting reflectors to bounce light, absorb light
Simple reflectors can be made of card, paper, cloth, board, polystyrene foam like packing foam from a dishwasher or flat pack furniture pack.
The whiter or more mirror like the more reflective. Just be aware that the colour of the reflector has an effect on the mood or colour of the subject. The reflector can cool the image down or warm it up or give it a colour hue.
Reflector performance can be affected by distance to the subject, distance to the light source and reflectivity and texture.
It is useful to have a variety of reflector sizes, for example a small reflector placed in front of and below a bonsai can fill in unwanted shadows and show more detail which may have necessarily been hidden without the reflector.
You can use lighting diffusers to soften and widen light. To spread a single light source and to reduce the intensity of light.
A lighting diffuser can be as simple as a curtain on a window a sheet in front of a window or
Please note that care should be taken using light modifiers near hot and cold light sources as there is a risk of fire/electrocution. Always use the appropriate non-flammable, non-conductive light modifiers for your choice of lighting. Reflectors should always be secured from being knocked or blown over by gusts or drafts.