Prison Visit from Robert Kraft & 76ers Owner

Huge day for Meek Mill ... the rapper got a special visit from TWO insanely rich and powerful guys -- Patriots owner Robert Kraft and 76ers owner Michael Rubin.

The two titans arrived together at Chester State Correctional Institution in Pennsylvania to spend some time with Meek who's serving 2 to 4 years for a probation violation.

Meek had been on probation stemming from a 2008 drug and weapons case -- but a Pennsylvania judge ruled his two arrests (an airport fight and a reckless driving incident) constituted a violation.

We know Rubin is a huge supporter of Meek -- he's visited several times before. He's also close with Robert Kraft, so we're guessing he was the driving force in getting…