It is written this way to create a condition.
Your example:
If the total number of items where TotalWeight has a value of 1 is < a value of 1
would apply the rule only if there where 0 (< 1) items in the cart that had a TotalWeight value of 1
Read it as:
If the total number of items where <condition>
<condition> =
Column TotalWeight has a value of 1 is < a value of 1