It's when the object comes first and the subject comes last. For example, the subject is underlined and the object is in blue:
Active: Max hit Sam.
Passive: Sam was hit (by Max).
Note, "by Max" is optional (...). It doesn't need to be stated.
Use the passive to place focus on the object.
There are three steps: 1) Move the object to the front, 2) change the verb to passive, and 3) add 'by' to the subject, like this,
Active: Max hit Sam
1. Move: Sam
2. Change the verb: Sam was hit
3. Add 'by': Sam was hit by Max.
Passive: Sam was hit by Max.