A tsukemono ishi 漬物石(or 漬け物石) is a heavy rock or stone used to apply pressure as part of the pickling process (tsukemono means ‘pickled things”. This can be translated into English as “pickling stone”, “pickle stone”, or “heavy stone”, etc. The stones can be natural rocks or can be store-bought (市販 ‘commercially available’). Here are some example:
(1) Simple rocks or stones used as weights (source)
ってくらいに漬物石を載っけます。 “Put on pickling rocks.”
庵治石でできた可愛らしい漬け物石 “pretty pickles made of Aji stone”
As seen below, this version has a rope in the middle to make it easier to use.
(3) Modern examples made from polyethylene and cement (source)
Note: this website spells tsukemono ishi as つけもの重石, using the hirigana つけもの in place of 漬物 on some product pages.
(4) ガラスの器 漬物石 glass pickle stone (source)