The meaning is roughly the same for both versions: you should arrive before 10, however there is a difference in when the two versions would be used.
before tends to be used when the action has no lasting effect. For example, if you have to pick up a document and then leave, but the person who has the document will be busy from 10am, then any time before 10am is acceptable, somebody would probably say:
Please come back before 10 am
by tends to be used when the action has some lasting consequence that will still be in effect at the indicated time. For example, If you have a meeting with somebody at 10am, you need to arrive before 10am, and you still need to be there at 10am, somebody would probably say:
Please come back by 10am
Another example for by would be if you have to submit a report before a certain date, because once a report is submitted it stays submitted.
The report must be submitted by 1st May 2017.
Parents generally tell their children to be home by 10pm (ie they stay in once they have arrived) rather than before 10pm.
Best Answer
"In two minutes" means in approximately two minutes, possibly more, possibly less, but not that much more or less. "After two minutes" means in at least two minutes, possibly a bit more.