Equipment rack with hollow tubes.

lostbears...Oh ok so it is for vibration more so for TTs... Thanks

In addition to damping ringing of the metal, it provides more mass, which many believe helps dampen vibrations and unwanted resonance. 
Sound Anchor uses sand to fill their very well-made steel stands.
Star Sound sells Micro-Bearing Conductive Steel™ ball-bearing steel fill material that they use in their own highly-regarded stands, and that they previously used in cables they made.
Core Audio Designs uses lead shot, clay, and sand in their racks.
Some people use lead shot alone, which I have never tried.  You would want to be sure the lead is used in a sealed chamber, such as the sealed legs of an equipment rack, so the lead is not exposed or allowed to migrate to areas where it could be ingested or inhaled by humans.

It is for Vibrations and damping it does work my Rix Rax is filled with Silika sand.Do this asap or ask more questions.

@elizabeth  Would it be worth adding the sand even though i dont use a TT. ? I dont have anything special as far as racks that i have. In fact they were very cheap. But i did go to a granite wholesaler and got 5 pieces of 2 in thick granite cut and using them as platforms for my ARC Ref 6 on one rack , another granite platform for my Rega Isis cdp on the other rack, a granite platform for my Krell FPB 600 that sits on the floor. And 2 granite platforms to put under my B&W 800 Matrix speakers. Also using for now Vibra cones and pucks for my preamp and cdp.