This is the 3645th most frequent Japanese word.
飲み込む
"飲み込む" means "to swallow" or "to understand/grasp."
In this sentence, 飲み込む is used metaphorically to mean 'to understand or comprehend'.
彼はその表現を理解して飲み込みました。
He understood and comprehended the expression.
Here, 飲み込む is used literally to mean 'to swallow' a liquid or something physical.
私は水を飲み込むときに喉が痛いです。
My throat hurts when I swallow water.
In this context, 飲み込む implies the act of accepting or embracing an idea or point of view.