Letters, sounds and spelling.
Letter Lab covers the groundwork of reading: knowing letters by name, knowing them in both cases, and knowing what sound each one makes. It moves from pure letter recognition up to picking out the sound a word begins with, which is where letter knowledge starts turning into reading.
The 2-4 level is letter finding and uppercase-to-lowercase matching only, with no sounds yet. The goal at this age is simply that letters become familiar shapes with names.