A thesis is doing its job when it passes four tests. It is arguable — someone could reasonably take the other side. It is specific enough to defend in the length assigned. It answers the prompt rather than the topic. And it signals the reasoning the essay will follow, so a reader knows where the argument is going. A statement that fails any one of these will usually produce an essay that drifts.
Too broad: “Social media has changed society in many ways.” Nobody disagrees, and “many ways” commits to nothing. There is no argument to defend, so the essay becomes a list.
Stronger: “Because algorithmic feeds reward outrage over accuracy, social media platforms have made political compromise harder to sustain than mass media did.” This is arguable, scoped to one claim, and the “because” clause tells the reader what the body paragraphs will have to prove.
Just a fact: “The New Deal created several federal programs during the 1930s.” True, and therefore not a thesis. A factual statement cannot be argued, only confirmed.
Stronger: “The New Deal's lasting significance lies less in the programs it created than in establishing the expectation that the federal government is responsible for economic security.” The claim ranks one effect above another, which is a position a reader could challenge.
Those are the criteria the checker applies: arguability, scope, prompt alignment, and whether a line of reasoning is visible. Fix the thesis first and the rest of the essay gets easier to write — and easier to grade. Once a draft exists, move into full essay grading against your rubric.