Net assets are sometimes the same as net worth, or shareholders' equity = assets minus liabilities. The term net assets is commonly used with charities or not for profit entities. Although these entities don't make money, it is important to maintain reasonable reserves to help future growth.
Additional meaning of Net assets:
In "Return On Net Assets" (RONA) it's often fixed assets plus net working capital (current assets minus current liabilities), which may be slightly less than total assets.
If net assets are negative, then an entity is balance sheet insolvent.