Currently, AI means Artificial Neural Network (ANN). That's only one specific approach. What ANN boils down to is one huge system of equations.
The file stores the parameters of these equations. It's what's called a matrix in math. A parameter is simply a number by which something is multiplied. Colloquially, such a file of parameters is called an AI model.
2 GB is probably an AI model with 1 billion parameters with 16 bit precision. Precision is how many digits you have. The more digits you have, the more precise you can give a value.
When people talk about training an AI, they mean finding the right parameters, so that the equations compute the right thing. The bigger the model, the smarter it can be.
Does that answer the question? It's probably missing a lot.