IHS is a frequently misunderstood Christian symbol. People will often attempt to guess what English words the letters stand for. The problem with this is that the letters are not English.
Some argue that the IHS symbol began similarly to IHC, with which it is easily confused.
Christians eventually started using these letters not only as an abbreviation of the name "Jesus," but also as something of an acrostic, to stand for the phrase "Iesus, Hominum, Salvator" - typically translated as Jesus, Savior of Humankind.
(There are differing opinions on the relationship between IHC and IHS and their intended meanings. I have chosen these interpretations as doing so provides multiple avenues for teaching.)
Much like IHC, this symbol can be found frequently on tables and vestments. I have also seen it as the center of a Celtic cross. It is often highly stylized, and may incorporate other imagery into its shape such as the Latin Cross as seen to the right. It may also be employed on its own, rather than as part of larger imagery.
Read Romans 5.18-21. How does this passage talk about salvation? What does it mean for Jesus to be Savior of humankind? What are we being saved from?
Next, Read Acts 4.1-13. When imprisoned and questioned, Peter and John talked about salvation. To whom do they connect salvation? What do they say about the name of Jesus?
This is another symbol that contains a confession of faith. These can be easy to preach in that they provide direct assertions. They can be somewhat confining, however, in that they proclaim precisely one assertion.
Nonetheless, it should not be difficult to preach that Jesus is the savior of humankind. In preaching this, you could talk about what exactly salvation means, who Jesus is, and why humanity needs salvation in the first place.
In short, this symbol offers a great option to outline issues of Christology and atonement.
For me, this symbol fits most readily in the season of Easter, though the season of Christmas also fits nicely if you intend to discuss issues of Christology. The message of this symbol is so central that it could fit almost anywhere in the liturgical calendar.