Somnia simulates your world while you sleep, instead of magically jumping to daytime, everything progresses. Mobs will spawn and despawn, machines will run, ores will smelt.
The multiplier value on the left (in orange) is the speed Minecraft is running at. The default value without this mod is of course ?x1? which is 20 ticks per second. You can calculate your theoretical TPS (ticks per second) value with, your multiplier * 20. This value will vary depending on how active your world is, how many mods and such you have and your CPU, Somnia does balance the multiplier to help prevent FPS drop in single player. The time value on the right (in white) is the current dimension time (prior to prerelease 1c, this always showed the overworld time since 1c introduced dimensional sleeping).
The configuration is pretty much self-explanatory. It allows you to define: a multiplier cap, a valid enter sleep period (when you can start sleeping), a valid sleeping period (the time period you can be asleep during), a maximum sleep duration (amount of time you can sleep consecutively, measured in ticks) and a toggle to allow sleeping with armor.