Refer to figure.
Airborne collision avoidance system (ACAS) indications shall be used by pilots in the avoidance of potential collisions, the enhancement of situational awareness, and the active search for, and visual acquisition of, conflicting traffic. The ability of ACAS to fulfil its role of assisting pilots in the avoidance of potential collisions is dependent on the correct and timely response by pilots to ACAS indications.

• When there is a collision danger between two aircraft, the ACAS systems of these aircraft will inform the pilots in the form of Resolution Advisory (RA) giving them instructions to follow. The ACAS systems of the two aircraft are communicating with each other in order to provide the pilots with non-contradicting instructions (i.e one aircraft may instructed to climb and the other to descend). Both pilots should immediately follow the given instructions.

ICAO Doc 8168 - Chapter 3
OPERATION OF AIRBORNE COLLISION AVOIDANCE SYSTEM (ACAS) EQUIPMENT

3.2 USE OF ACAS INDICATORS
c) in the event of an RA, pilots shall:

1) respond immediately by following the RA as indicated, unless doing so would jeopardize the safety of the aeroplane;

Note 1.— Stall warning, wind shear, and ground proximity warning system alerts have precedence over ACAS.
Note 2.— Visually acquired traffic may not be the same traffic causing an RA. Visual perception of an encounter may be misleading, particularly at night.