Operators is one of the ten categories of default Scratch blocks. They are color-coded light-green and are used to script math equations and string handling.
The block checks if the first value is less than the second value. If it is less, the block returns true; if not, it returns false. This block works with letters too, as well as numbers. In Scratch, letters at the top of the alphabet (e.g. a, b, c) are worth less than letters at the end (e.g. x, y, z).