behind or after: the place at the back of something, or the time that comes later
This is one of the most common little words in the Hebrew Bible, and almost always a relational word rather than a thing in itself. Its single idea is being on the far side of a reference point: behind it in space, or later than it in time. Picture standing at someone's back. From that one picture the whole range unfolds.
In space it means behind or at the back of: a person stands behind a door, a sword strikes from behind, a flock grazes behind the shepherd. From standing behind comes the idea of going behind, that is, following: troops chase after a fleeing enemy, a servant walks after a master. Hebrew then stretches that following image into a picture of loyalty. To go after someone is to give them your allegiance, which is why the Bible's gravest warnings are about going after other gods, and its highest praise is for those who follow the LORD wholeheartedly.
In time the same word means after or afterward. As a connector it introduces what happens once something else is done (after the flood, after these things), and standing alone it can simply mean later. The spatial and the time senses are not two separate words but two views of one idea: what lies behind you in space is also what you have already passed in time.
Readers should not press a heavy meaning onto a word that is usually just doing the quiet work of placing one thing behind or after another. The weight falls on the phrase, above all the loyalty phrase to go after, not on the word by itself.