keep one's ear to the ground

この表現は、文字通りですと、地面に耳を付ける、ということになります。

地面に耳を付けるのは近づいてくる敵（！？）の接近を知るためかもしれませんし、地震の予兆かもしれませんが、自らの身を守るために情報収集をする、ということが目的です。

このことから、社会などの動向に注意関心を持つ、といったような意味で用いられます。

Collins Cobuild Dictionary of Idiomsの定義を引用します。

If you keep your ear to the ground, you make sure that you find out about the things that people are doing or saying.

"keep"という動詞以外に、"have"が用いられることもあります。

For healthcare entrepreneurs and investors, having an ear to the ground on healthcare policy disrupters and priorities is valuable for anticipating trends, challenges, and opportunities for healthcare innovation now and in the future.

(Contributed: A Biden health policy retrospective: Implications for healthcare innovation. Mobile Health News. September 23, 2021.)

もうひとつ、コーパスから。

Where other metropolitan chief executives take care to maintain their political viability, Fenty keeps no close political counsel, putting his faith in his own sense of what "regular people" think. But Fenty either did not keep his ear to the ground or misunderstood that people often tell you what you want to hear.

(Washington Post, 2010)