Re: (with a colon) means regarding, on the subject of. Often we can find it in the subject line of an email.

Re: Your enquiry for USB cables

With the same meaning, re can be used in informal language:

We need to have a meeting re the latest sales report.

You can read more about regarding here.

In many words the prefix re means again:

Two years later he remarried.
The votes had to be recounted.
After the installation you have to restart your computer.
All components are reusable.

Since re here means again, you must avoid writing He remarried again or The votes had to be recounted again (that would mean that he married at least three times or that the votes were counted three times). You can read more about unnecessary words here.

Re can also mean a change in the position or state of something:

relocate = locate in a new place
rearrange = arrange in a different way

The image shows a room with bookshelves. There are many book on the floor, on a desk and on a step-ladder. The purpose is to illustrate the verb rearrange in a post about the prefix re..
He decided it was time to rearrange his book collection

Some words with re have two versions, one with a hyphen and one without, and there is a difference in meaning.

recollectrememberre-collectcollect again
recoverget back health, ability,
possession, etc.
re-covercover again
reformchange or improve somethingre-formcreate again
represssubdue, not allow feelings,
etc., to be expressed
re-pressmake a new copy of a recording
resentdislike or be annoyed at
someone or something
re-sentas in 'He re-sent the parcel'
reservearrange for something to be
kept for your future use
re-serveserve again

Use a hyphen if re means again and if omitting the hyphen would cause confusion with another word.

You can read more about using a hyphen here and about the difference between a hyphen and a dash here.