Definition
A claim is a specific statement that a debater is committing to defend. It is not a topic and not a vague direction, it is a sentence someone could directly agree or disagree with. In the standard claim, warrant, impact structure, the claim is what you are arguing; the warrant is why it is true; the impact is why it matters.
Example
"Homework" is a topic. "Mandatory daily homework harms younger students more than it helps them" is a claim. The second version can actually be argued for or against, because it commits to a specific, checkable position rather than gesturing at a general subject.
Common mistakes
New debaters often state a claim so broadly that it is impossible to defend cleanly, or so hedged with qualifiers that it says almost nothing. A claim like "homework might sometimes have some negative effects" gives an opponent nothing solid to engage and gives a judge nothing solid to vote on. Write your claim as a single sentence you could defend against a direct question. If it will not compress that far, it is probably two arguments tangled together and should be split.