Regarding idol worship, there are certain activities that are considered to be objectively an act of worship and will be forbidden, while other activities may be specific and limited to a certain type of idol. Generally speaking, a person will be held liable for avoda zara – the prohibition against idol worship – when he performs any one of a number of acts of worship. These activities include commonly used methods of veneration including sacrificing or burning incense, offering a libation or bowing down, and even simply saying “you are my god.” Other types of obsequiousness, such as hugging and kissing the idol, washing or cleaning it and so on would be forbidden, but would not serve as true idol worship.
There are other modes of worship that ordinarily would not constitute an act of avoda zara, except with a specific idol or deity for which that act is a unique form of worship. Thus ha-po’er atzmo le-ba’al peor – someone who relieves himself in front of the idol Peor – or ha-zorek even le-markulis– someone who throws a stone to the idol Markulis – will also be held liable for performing an act of avoda zara, since this is the unique method of worshiping these idols. The Gemara on today’s daf brings the opinion of Rabbi Elazar who derives from a passage in Sefer Vayikra (17:7) that sacrificing to Markulis is also considered to be avoda zara, even though that is not the normal method of worshiping that idol.
Markulis is the name given by the Sages for the Roman god Mercurius (Mercury), who was also known as the Greek deity, Hermes. Among his many responsibilities, Mercurius was the patron of the highways and travelers. This position led many to erect statues of him on crossroads. Oftentimes, these representations presented just the head of the idol and passersby would place stones at the foot of the statue. On occasion the representation was simply a pile of rocks, and travelers who passed by the pile would toss their own stone on it as an offering to the god.