A loom is a mechanism for weaving two sets of thread together to form cloth. The simplest looms date from prehistory; several types of loom were used throughout period. Although a period artifact, we’ve no examples of the loom used as a period heraldic charge.
The illustration is of a harness loom, which uses foot pedals to raise and lower the harnesses holding the heddles. It would be used by a human facing dexter, which makes this depiction the default orientation. Harness looms might have one of several designs; the depiction is taken from Jost Amman’s Book of Trades, 1568 .
Diana Doria bears: Azure, a harness loom argent.