1 John 2:29-3:6; Ps 98(97):1. 3c-4. 5-6; John 1:29-34
If you look closely at many priestly vestments, and at many Chalises, Bibles and other holy items, you will find there the monogram made up of stylised letters I, H and S. This 'Christogram' is ancient, and stems from the greek spelling of the name of Christ, by abbreviation or shortening of Jesus' name in Greek to the first three letters. Thus ΙΗΣΟΥΣ, ιησυς (iēsus, "Jesus"), is shortened to ΙΗΣ (iota-eta-sigma), sometimes transliterated into Latin or English characters as IHS or ΙΗC.
The memorial day for The Holy Name of Jesus was set in 1530 but veneration of the holy name certainly existed long before that, as ancient inscriptions of the Christogram in various forms exist. By invoking the holy name we remind ourselves that we are baptised into Christ - we adopt his name when we adopt the white garment, receive the candle, and are washed in the water of the font - we are Christians.
If it helps, you can remember the symbol as "In His Service" or Iesus Hominum Salvator ("Jesus Savior of Men") but contrary to some commentaries, those are NOT the origin of the IHS Christogram - they derive from it.