〜とは限らない
not necessarily, not always
Plain form + とは限らない
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とは限らない says something isn't always true, heading off a generalisation: 高い物がいい物だとは限らない, "expensive things aren't necessarily good". 限る means "to limit", so it's literally "it isn't limited to".
It often pairs with 必ずしも ("necessarily") or いつも, and with からといって: 日本人だからといって、日本語を教えられるとは限らない.
The polite form is とは限りません.
It's close to わけではない. とは限らない is about exceptions to a general rule ("not always"), while わけではない corrects a specific misunderstanding ("it's not that").
It's a favourite of essays and opinion pieces, where a writer questions an assumption before making their own point. In conversation, it's a polite way to disagree: you don't say the other person is wrong, only that it isn't always so.
Examples
高い物がいい物だとは限らない。
Expensive things aren't necessarily good.
日本人だからといって、日本語を教えられるとは限らない。
Just because someone is Japanese doesn't mean they can teach Japanese.
天気予報が必ず当たるとは限らない。
The weather forecast isn't always right.
有名な店がおいしいとは限らない。
Famous restaurants aren't necessarily good.
お金持ちが幸せだとは限りません。
Rich people aren't necessarily happy.