When using configurable products, there are multiple simple products. Each simple product has its own stock. A configurable product does not have stock and is really a virtual product. It's not an actual product but an instance that can be shown in the product list. Customers will always have to select a simple product when purchasing.

Go to: Stores > Settings > Configuration > Catalog > Tweakwise. In the Export section, look for Combined product stock calculation mode.

These options are available:

Sum: the sum of the stocks of all the simple products (almost always suitable)
Maximum: the simple product with the highest stock will be used
Minumum: the simple product with the lowest stock will be used