The amount of tokens that a blockchain protocol locks, and potentially holds subject to Slashing, as a punishment of Validators who misbehave.
In other words, it is the cost that it would take to bribe or control the majority of the Validators of a blockchain network. If one can successfully do this, they can essentially control that network. The higher this number is, the more secure a protocol is.
Economic security is the main property that delivers verifiable execution, one of the pillars of Trustless blockchain networks, and plays a key role in other properties too. We trust that the blockchain executes its STF correctly, because we know validators of a said blockchain have billions of dollars of tokens at stake, and game theory informs us that they will not risk an incorrect execution of the STF knowing they will get slashed.
As the economic security of a network increases, the the confidence in its Trustless-ness increases.