In plain words
Swapped letters, an added hyphen or a different ending (.co for .com) make an address that looks like yours at a glance.
Why it matters to a firm
You cannot stop someone registering one, but you can find it early. A lookalike that is set up to send email is often the first sign of a forged invoice.
How anyone can check it
Type two or three misspellings of your domain into a domain registration search and see which are taken.