To quote the official site here as they sum up what these are best: “Sentiments are those special fuzzy feelings that form between Sims when they share a memorable moment together.
New half walls are here and there are 12 additional heights to now choose from- up from the seven that were there originally. This is a huge update and we have barely even scratched the surface of all you can now do. You can customise the look of them with the help of the Platform Trims. Platforms will be able to be stacked and you are encouraged to do so and get creative in the process. Within a story level, you will now be able to build rooms at different heights with the platforms update- or even the reverse with them not being able to be constricted underground- deep under by the sounds of things.
As for what is included, we will cover the gist here but head on over to the official EA Sims site to read everything- there is a lot to cover in a lot of detail! The update is live now, having taken place on November 10th, so you should have the update triggered when you log into the game.