Full Idea
We explain truth for sentences in terms of satisfaction of formulae. The crux here is that for a sentence, either all sequences satisfy it or none do (with no middle ground). For formulae, some sequences may satisfy it and others not.
Gist of Idea
Truth for sentences is satisfaction of formulae; for sentences, either all sequences satisfy it (true) or none do
Source
William D. Hart (The Evolution of Logic [2010], 4)
Book Reference
Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.107
A Reaction
This is the hardest part of Tarski's theory of truth to grasp.