ETA means Estimated Time of Arrival: the predicted time an aircraft, vehicle, or shipment will reach a destination such as an airport, gate, or reporting point. It is calculated from schedule, route, and live conditions (weather, traffic, speed) and is often updated in flight.
ETD means Estimated Time of Departure: the forecasted time the flight or shipment is expected to leave its origin (gate or runway). It starts the operational timeline and is revised for delays such as weather, congestion, or ground handling.
In short, ETD is about when you leave; ETA is about when you arrive. A later ETD usually pushes the ETA later as well. Related terms include ATA (actual arrival) and ATD (actual departure).
Source: ETA (Estimated Time of Arrival) — TarmacView aviation glossary.