开灯 (kāi dēng) means “turn on the light”

More examples

天黑了，我该开灯了。tiān hēi le wǒ gāi kāi dēng le

It has grown dark, so I should turn on the light.

The movement from darkness to usable light by activating a lamp or lighting device.

开kāi: Switches on power or a device to make it work.
打开dǎ kāi: Operates a switch or knob to start a device.

把灯或照明设备打开，使其发出光。bǎ dēng huò zhào míng shè bèi dǎ kāi shǐ qí fā chū guāng

开灯 (kāidēng, "turn on the light") joins 开 (kāi, "start or open") to 灯 (dēng, "lamp"), presenting the lamp as something brought into operation.

Chinese often leaves the particular lamp or lighting device understood from the situation, so this compact verb phrase is natural when the room makes the object clear.