The manual for the car, which you can look at on the main screen does.
Page 52 of the owners manual, at least in the version I currently have on my hard drive, under CAR STATUS:
==========================
Note: In cold weather, some of the stored
energy in the Battery may not be available
until the Battery warms up.
When this
happens, a portion of the Battery meter is
blue and the driving distance value has a
snowflake image next to it. If Model 3 is
plugged in, you can heat your Battery
using wall power by turning on climate
control using the mobile app.
When the Battery warms up, the blue portion
on the meter and the snowflake image are no
longer displayed.
============================
Also, if I go to a popular search engine like google and type "Snowflake icon tesla model 3" I get pointed to several threads that explain exactly what it is.
OP, if you have not read the manual yet and are a new owner, I suggest reading it. There is a lot of good information there, and every icon that appears on your screen is covered in some capacity in the manual.