These settings are in the chat options panel and change how chat looks for you (and only you), with the exception of your username color.
Username color picks the color your name appears in for anyone with username colors enabled. Display username colors turns everyone else's colors off if you prefer.
Chat width, Message display (Cozy or Compact) and Chat font are self explanatory. Zoom level scales the whole chat, and Chat font size only the text.
PMs display decides where private messages appear: in their own Tab, Inline with the room, or Both. Tab is the default.
Max custom emote size caps how large custom emotes render. Always animate GIF emotes plays animated emotes all the time; with it off, an animated emote is frozen until you hover over it.
Show raffle confetti is on by default, playing a little animation when a raffle ends. Disable timestamps does what it says on the tin.
Play sounds has three options: On new messages, Only on messages mentioning me (the default), or Never play sounds.
Play sound only when chat is not focused is on by default, so you don't get a sound for rooms you have selected.
Three switches control browser notifications for the chat: mentions (off by default), @role mentions and @everyone mentions (both on).
Four switches to control status messages: joins and leaves (off by default), bans and topic changes (on).
Show shared-IP indicators turns the indicators on or off. It appears only for moderators and up, and the indicators never render in an on-stream overlay. Each indicator has a random hash code and color and if two users share one, that means they are connecting from the same IP address. Sometimes this is benign, like husband and wife sharing their home network, but it can also reveal sockpuppets.