Hello Nadia,
You can use something like if you want to display post items with the same category:
[recent_posts count="3" category="articles"]
The offset is useful if you want to display 8 or 12 post items of the same category. For example:
[recent_posts count="3" category="articles"]
[recent_posts count="3" category="articles" offset="4"]
[recent_posts count="3" category="articles" offset="8"]
The offset is needed in displaying 12 items because you can only display up to 4 items in each recent post element.
Hope this explains it briefly.