What is the complete thought?

a complete sentence has a subject (noun),predicate (verb), and it has no dependent clauses.

EX.1 I went to go eat lunch with my boyfriend.

(correct)

EX.2 While I went to go eat lunch with my boyfriend.

(incorrect)

Example 2 is wrong because it doesn't say what happened while I went to lunch. Example 2 has a dependent clause which is while.