Tyrion was lying to Dany about the bells meaning surrender. Jaime (the "stupidest Lannister") later catches out Tyrion's lie, but it takes him a few seconds. You can see his brain processing it.
Way back in 2.09 "Blackwater" - the last time the city was under attack - Varys says he hates the bells because they always sound for horror - the death of a king, a siege. Tyrion adds "and weddings" and Varys says "exactly," which was funnier at the time because we hadn't had the Red or Purple weddings yet.
Then out in the bay, Davos's son hears the bells and suggests that the city is welcoming Stannis as its new king. Davos says he's "never known the bells to mean surrender", and he calls for the bells to be answered by war drums.
We also hear the bells while Varys is waiting for the crate containing Tyrion to be smuggled onto the ship as they escape King's Landing in 4.10 "The Children." and we have a symbolic use of "bell means horror" in 6.10 "The Winds of Winter" when the Great Sept's bell crashes into the street. The bells show up often enough we can call it deliberate symbolism.
Tyrion is telling a lie to Dany about the bells because he at least wants the people in the city to have fair warning to take cover before she begins her attack (and maybe if Dany buys the lie - which I don't think she does nor did Tyrion think she would - he could stave off the horror almost entirely).
The more interesting question might be - what does this tell us about Dany's state of mind? It's worth rewatching the dialogue portions of the episode while keeping in mind that Tyrion is lying. It changes our perspective of those scenes. It's important that Dany was just talking about mercy before Tyrion brings up the bells; and afterward she gives Tyrion the means to carry out his plan by telling him about Jaime.
