I saw a video that said to fill cracks and holes with a paste made from the wood dust mixed with resin. I was wondering, can somebody tell me, what is resin?

From Wikipedia:
Resin is a hydrocarbon secretion of many plants, particularly coniferous trees, valued for its chemical constituents and uses such as varnishes, adhesives, as an important source of raw materials for organic synthesis, or for incense and perfume. Fossilized resins are the source of amber. The term is also used for synthetic substances of similar properties.

Synthetic resins:

Synthetic resins are materials with similar properties to natural resins—viscous liquids capable of hardening. They are typically manufactured by esterification or soaping of organic compounds. The classic variety is epoxy resin, manufactured through polymerization-polyaddition or polycondensation reactions, used as a thermoset polymer for adhesives and composites. One more category, which constitutes 75% of resins used, is unsaturated polyester resin. Ion exchange resin is another important class with application in water purification and catalysis of organic reactions. See also AT-10 Resin, melamine resin.

Us pros don't typically do it that way unless we have to, because we cannot get a wood filler that will match the wood well enough.

Thanks !!!