Someone said "Your team kicking serious butt" to me in an email. Does that mean that he thinks my team is awesome? I feel it may have a similar meaning to "kick ass" but I'm not sure and can't find this in dictionaries. Hope someone can help to clarify. Thanks.
It means that your team is decisively winning its matches. "Kicking butt" and "kicking ass" both mean winning a fight. "Serious" doesn't mean solemn here, and it doesn't really modify "butt." It's just a slang intensifier meaning "in an enthusiastic manner."
The phrase "Your Team is kicking serious butt" is generally used as a softer, more workplace appropriate way, of saying "Your team is kicking serious ass."
The phrase "kick ass" is a longstanding idiom that generally means someone is doing something really well, excelling, or, in your case, being awesome.
If you're looking for more information about it, thefreedictionary describes the phrase "kick ass/kicking ass" as:
kick ass/butt Vulgar Slang
To take forceful or harsh measures to achieve an objective.
and oxforddictionaries.com describes it similarly:
Definition of kick some ass or butt in English:
North American vulgar slang
Act in a forceful or aggressive manner.
for information about the etymology of the phrase kick ass (if you're curious) this stack exchange link might be helpful: What is the etymology of "…kick ass and take names"?
It means your team is doing really well (as compared to other teams)