To apprehend, as we all know, is to understand or grasp the meaning of something.
The kids apprehended the lesson very well.

To apprehend, as very few would know, is also to get arrested or to be taken into custody by a legal authority.
The burglars were apprehended by the police in no time.

Apprehension, besides meaning the act of apprehending (understanding), also means anxiety or fear of future trouble or misfortune.
There was a lot of apprehension in his mind before opening the telegram.