This feature is still in development and the details have not been finalized.
Energy (NRG) allows usage of the Subsocial network without tokens, and has a few key points and implications. Energy is not actually a token, and cannot be transferred between accounts. To create energy, tokens need to be burned, but the energy can be created in a different account than the one burning the tokens.
For example, Alive burns 10 SUB and creates 11 NRG in her account. This allows Alice to perform more actions than if she had used her SUB tokens for the fees, so she is incentivized to convert her tokens into NRG, creating deflationary pressure on the token.
Alice could instead burn those 10 tokens and create the 11 NRG in Bob's account, allowing Bob to use Subsocial without ever getting SUB tokens.
This important feature allows new users to onboard to Subsocial much easier, and will be covered more in the Applications section.