The base of BIM (Building Information Model) is a digital 3D model of the building. It consists of walls, slabs, roofs, doors, windows and such, just like the real building.
In BIM we also assign invisible data (metadata) to the digital elements of the model, such as structural function, fire resistance value, acoustic performance value, thermal transmittance, price, weight, product information, etc.
The model and the metadata together build a complex database we call BIM.
The information stored in BIM can be exchanged between the stakeholders during the design and construction phases of the project and during the whole lifecycle of the building (operation, reconstruction, repurposing, demolition).
Archicad is a BIM software tool that helps architects build the BIM of the designed building and also manage and exchange the information stored in BIM in flexible ways.
You can read more about how building information is structured in Archicad in this article.
If you are looking for a more generic explanation of BIM in layman's terms, watch this video: