"Portable alpha" refers to separating the active manager’s excess return from the base market return and transporting the alpha to some other market index.
For example, to accomplish this using futures, the investor allocates a pool of capital across three strategies: The majority of the assets are invested with the active manager, a small portion is used to purchase the "other market" index futures, and index futures are sold to eliminate the market return (beta) from the active manager’s total return. The investor is then left with the active manager’s alpha plus passive exposure to the "other market" returns.
Another example for an active bond investment...
Invest in an actively managed fixed income portfolio & keep the exposure to both the fixed income beta and the manager’s alpha. Then use the fixed income portfolio as collateral for a portfolio of futures or swaps that provide equity exposure.
Note that the beta exposure, say through a third-party overlay manager buying futures or executing swaps, may require only a small portion of cash/collateral (for margin). This may allow for a greater portion of the funds to be committed to generating alpha.
