We may not sit—and according to some opinions, even lean—within a radius of four amos (approx. 6 feet or 1.92 meters) in any direction of a person who is davening Shemoneh Esrei. There are various reasons given, and one of them is that the area around a person who is davening Shemoneh Esrei is considered admas kodesh, holy ground, and sitting idly next to such a person is disrespectful to the presence of the Shechina, the Holy Divine Presence. According to various opinions, this also applies to a person who is saying the first posuk, verse, of the Shema, and a number of poskim even extend it to someone reciting Kaddish. If we are davening ourselves, any part of davening, including shiros ve’tishbachos, songs and praises to Hashem, we may sit near someone who is davening Shemoneh Esrei. Likewise, if we are verbalizing words of Torah, or learning from a sefer—even without saying the words—we may sit. However, if we are merely reviewing Torah thoughts mentally, we may not sit near someone who is davening Shemoneh Esrei. If it is during Chazaras HaShatz, the chazan’s repetition of Shemoneh Esrei, some poskim hold that if we are paying attention to Chazoras HaShatz and following along in the sidur, we may sit near someone who is davening Shemoneh Esrei.