Empress Pulcheria (pi 2145) was the sister and regent of Theodosius II and the cause for Marcian's elevation through a political marriage which gained him imperial authority and guaranteed her continued virginity. This stele is briefly mentioned at sigma 1084.
On her see McClanan pp. 21-25 and the De Imperatoribus Romanis articles on Theodosius II (web address 1) and Marcian (web address 2).
[1] Most likely a statue, but the term could refer to other sorts of monuments.
[2] A gate of the Great Palace. See the map at web address 3.
[3] Or perhaps "next to the palace, by the peripatos." There is some doubt as to whether one or two stelae (statues) are indicated here. See the discussion of the corresponding passage from the Parastaseis syntomoi chronikai in Cameron & Herrin, pp. 207-8; compare Preger's critical edition of the Parastaseis syntomoi chronikai (p. 13) at web address 4. The word "peripatos" here seems to refer to a specific promenade or covered walkway in the Palace complex, for which see Maguire, p. 258 (web address 5).