The first is forward guidance, which works both by anchoring expectations about the level of future short-term rates, and by reducing the volatility around that level. However, forward guidance is only credible over the horizon of visibility of the monetary policy committee.
If the central bank intends to steer interest rates at maturities beyond that horizon, it has to do so by directly targeting the term premium through private or public asset purchases. As our short-term rates have fallen towards the lower bound, and hence the signalling power of rate movements has decreased, the ECB has progressively resorted to both these instruments.