You touch a willing creature to grant it the ability to see light in the infrared spectrum out to a range of 120 feet. Thus, the target can see differences in heat wave radiations for the duration. It has advantage on its Wisdom (Perception) checks that rely on sight (+5 bonus to its passive Perception score) to spot hidden creatures within its infravision range. It also has advantage on Wisdom (Survival) checks made to track a warm-blooded creature, provided the track is recent. Warm or cold creatures within range usually gain no benefit from obscurity (i.e. dim light, darkness) or invisibility when attempting to hide, at least with regards to the target of this spell.

However, the DM has great latitude in ruling which creatures can be seen with infravision in various circumstances. Certain incorporeal undead or elemental creatures such as air elementals or invisible stalkers might not be visible on the spectrum. An earth elemental standing motionless against a stone wall might be difficult to perceive. Cold-blooded snakes moving along the cold ground of the Underdark (therefore only slightly warmer than the background) might even succeed in surprising the target. 

Strong sources of infrared radiation (fire, torches, lanterns, or other heat sources) tend to emit infrared "shadows", so the infravision doesn't grant any benefit for spotting creatures in areas of bright light created by such sources. It actually makes it harder to spot these creatures. A creature with infravision is blinded until the start of its next turn if it starts its turn in an area of heated bright light.

While the target has infravision, it can't benefit simultaneously from darkvision. The spell doesn't allow the target to see light in the infrared spectrum of creatures or objects on the Ethereal Plane.

Finally, this spell has no effect in the bright light of daylight.