A block instance is a unique item created from a definition. When you place an instance, it inherits all of the data saved in the definition.
Block Instances are Points that inherit the geometries & properties from the Block Definition. You can have many instances of the same block definition in your project.
Instances include:
A geometric point, which attaches to the block basepoint
The child geometries saved to the definition (treated as a reference)
Properties that propagate to the child geometries based on settings in the properties palette