You use the word "by" to indicate a deadline.
You would use "by" in this way:
We're presenting this on Monday morning, so remember to get Henry's approval by Friday afternoon.
"Before" is similar in meaning to "by", but "before" doesn't carry the same meaning of "deadline". Instead, it's used when you need to do things in a certain order:
Remember to get Henry's approval before you present it to the client.
This is something you would say if a coworker is going to make a proposal to a client, but she is supposed to ask Henry, who's the Director, for permission first.