What is this feature on a Sukhoi Su-27?
It's the sensor of the IRST (InfraRed Search and Track) system.
An IRST allow an aircraft to search and track aerial or ground targets using passive infra-red radiation only thus reducing the probability of being discovered (for example using a Radar Warning Receiver).

The Su-27 can use it's IRST to track a target at very long distance and engage it using IR homing AAMs like the R-73 "Archer" or the R-27T / R-27ET "Alamo" without using its radar.

Nowadays, many current fighter aircrafts use some sort of IRST, for example the Eurofighter Typhoon.