What is it?
It is the difference of,
‘Our team defeated the visiting team.’ (Active)
‘The visiting team was defeated by our team.’ (Passive)
While there is nothing wrong with a passive sentence, they should be used sparingly and you can use the grammar check on your computer to hunt them down and exterminate. (Notice the passive ‘should be used’ in this sentence.)
Why should you do this?
If you have ever read a story, made up entirely of passive sentences you will understand.
It is like having to read or listen to Vogon poetry. (See ‘The Hitchhiker’s Guide to the Galaxy’)
Warning: Too many passive sentences may cause excessive drooling and in rare cases, epileptic seizures are possible. (On the other hand, perhaps it just feels that way.)