das war es, was ich

that was what I

→ das
phrase Rare #47,191
that was what I Introduces the thing or action the speaker intended, wanted to say, wanted to do, or wanted to know. It is usually followed by a verb in the past tense or another phrase completing the sentence.
GE: Das war es, was ich dir sagen wollte.
EN: That was what I wanted to tell you.

Das war es, was ich machen wollte.

That's what I wanted to do.

— GoodFellas (1990)

Das war es, was ich ihnen immer sagen wollte, aber ich habe es nicht gewagt.

That's what I always wanted to tell them, but I didn't dare.

— Ikiru - Einmal wirklich leben (1952)

Das war es, was ich wissen wollte.

That was what I wanted to know.