What do the tail numbers on aircraft mean?
TL:DR, SB-023 means it is airframe number 23 of the SU-30 model MKi. It is flown by the Indian Air Force.

The small rectangle is what is known as a fin flash. It generally identifies the country of origin of the aircraft and sometimes the garrison or command within that country. In this case it indicates India (not to be confused with the Irish Fin Flash. We can be sure it is India because the roundel further forward is that of the Indian Air Force.

The letters and numbers above are likely to be the "tail code" Which is a USAF term. I'm sure this term has a unique name in Indian military parlance but I can't find it. The Tail code usually describes the either the home base or  manufacturer of the aircraft as well as sequence number of the aircraft. Note that sequence number and serial number are not identical and multiple air frames may share the same sequence number as air frames are retired or refitted.

On a USAF Aircraft a two-letter code generally indicates its home base. The plane would then have a numerical code indicating year built and sequence number.

I've had a look through google images and every SU-30MKi I can find has an SB prefix to its tail code so I am going to guess that it is an indicator of manufacture and/or sub-type. Interestingly, all SU-30s and Mig-29s bear the same prefix within type (SB and KBU, respectively) but Mig-21s have a variety (I've seen UI, CI, CII, and CUI.) Which makes sense since India has fielded multiple variants of the 21 but only one variant of the 30 or 29. My observations are based on a survey of the first 4 pages of google images for each aircraft so are hardly exhaustive.

The Sequence numbers come up in discussions among enthusiasts for example.

It is worth noting that India's military designation system evolved while the country was part of the commonwealth during WWII, the time when most of the world's modern designation systems were conceived. Despite that, India employs many Warsaw pact and a few home-grown systems, resulting in a unique combination of designation system and technologies.