In astronomical interferometry, the correlator is the device or software that interferes the waves to produce the interference pattern.
At visual wavelengths, interference is typically done optically, but at longer wavelengths, the signal can be digitized to the point where digital signal processing in hardware or software can do the work. It may be done in real time, or for very-long-baseline interferometry, it may be done later, after bringing together the signal data, consisting of digital files including very precise timing.
For most directions (those not perpendicular to a straight line between the antennas), the waves reach the antennas at different points in time, the antenna further from the source receiving each wave crest last, the period of time being termed a geometric delay. The identical delay can be imposed on the signal received by the nearer antenna using electronics, and the correlator can amplify those signals (voltage levels) that are received by both by multiplying the two. The timing of the rise and fall of the resulting signals as the Earth turns (due to the receiving antennas' most sensitive aiming directions passing the actual direction of the object) offers the most precise data regarding position in the sky, i.e., is what yields the extremely high angular resolution: angular distances are derived from the time periods over which the changes occur.