The word "heart" shows up 6 times in this 14 line sonnet so it becomes one of the most dominant images in the poem. Unfortunately, the heart images we're working with here aren't the cute, paper, cutout kind that get plastered all over Valentine's Day cards. When the speaker of Sonnet 133 talks about hearts, something horribly violent is usually happening to them (like, say, getting wounded or imprisoned). Oh, yeah, the mistress's heart is usually the thing doing all the damage. What's going on here? Wounded and imprisoned hearts are metaphors for the gut-wrenching emotions of being in love (or lust, we're not quite sure). When the speaker talks about "hearts," he's trying to describe the pain of being in a romantic relationship that's doing more damage than good.
Lines 1-2: Instead of coming out and saying something like "Damn you for hurting the feelings of me and my friend," our speaker personifies his mistress's heart and accuses it of "wound[ing]" both his and his buddy's hearts. Shakespeare's borrowing from the courtly love tradition, where lovers are often said to have been wounded by Cupid's arrow. But, Shakespeare kicks things up a notch by suggesting that the speaker's heart "groan[s]" in agony from a physical wound. Gross, right? It's also super-effective at conveying our speaker's sense of emotional pain and suffering.
Line 9: This is where the speaker imagines that his heart is a prisoner inside his mistress' heart, which he calls her "steel bosom's ward." There are a couple of things going on here. First, he's saying that he feels like a prisoner of love. (Yep, that's another courtly love metaphor and we talk about it more in our discussion of "Prison.") Second, he's telling his mistress that her heart (a.k.a. her emotions) are as harsh and cold as a "steel" jail cell ("ward").
Lines 10-11: This is where our speaker imagines that his heart is also a little jail cell that's got his friend's heart locked up inside. In other words, the speaker is fantasizing about keeping his friend from being emotionally hurt by the mistress. (He says he wants to be "his guard.") We can't decide if this is a sweet idea or just a creepy idea. We're all for protecting our friends from being hurt by other people, but we don't think it's healthy to sit around imagining that we've got our friends' hearts on lockdown. Go to our discussion of "Prison" if you want to think about this some more.