What does it mean to conserve natural resources?

To conserve natural resources means to manage and protect them to ensure their availability for the present and future generations. Let's evaluate the given options:

• A. waste them or use them up completely. This statement describes the opposite of conservation. Conservation aims to prevent waste and depletion.
• B. protect plants and animals from loss or danger. This statement accurately reflects a key aspect of conservation. Protecting biodiversity, including plants and animals, is fundamental to conserving natural resources and maintaining ecological balance.
• C. force animals and plants to become endangered. This statement is incorrect and goes against the principles of conservation, which seek to prevent species from becoming endangered or extinct.
• D. make them not be available for future generations. This statement is also incorrect. A core goal of conservation is to ensure that natural resources are available for future generations through sustainable practices.

Therefore, the most accurate definition among the choices for "to conserve natural resources" is to protect living organisms from harm and loss.

The final answer is $\boxed{\text{B}}$