I've seen 'em come, I've watched 'em go

from Dr. Dre – The Watcher Lyrics on Rap Genius

Meaning

Most likely this is a tribute to Eazy E (who died in 1995) and 2pac (who died a year later) — people who released their first albums after Dre started his career, and didn’t live to see his second album.
In general this line expresses how Dre has been making songs for a while and how other rappers started after him but are gone already.

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