For Orthodox, the hymn is sung as the last dismissal hymn of daily Vespers during Great Lent.
The entire hymn in Greek reads:
Ὑπὸ τὴν σὴν εὐσπλαγχνίαν,
καταφεύγομεν, Θεοτόκε.
Τὰς ἡμῶν ἱκεσίας,
μὴ παρίδῃς ἐν περιστάσει,
ἀλλ᾽ ἐκ κινδύνων λύτρωσαι ἡμᾶς,
μόνη Ἁγνή, μόνη εὐλογημένη.
In English:
Beneath thy compassion,
We take refuge, O Mother of God:
do not despise our petitions in time of trouble,
but rescue us from dangers,
only pure one, only blessed one.
Read more about this hymn http://www.newliturgicalmovement.org/2011/02/sub-tuum-praesidium.html.
The link includes a You Tube video of Bortniansky's beautiful setting of this hymn, which we sing here at St. Mary Magdalene Church.