It means "not a big deal". It is used as a phrase for telling someone that the particular thing they've done/mentioned isn't important.
Meg is calling her friends from Okinawa because a typhoon hit last night.
- Meg : Oh my God, I hared there was a big typhoon, are you okay?
大変だったね、大きな台風が来たらしいけど、大丈夫？ - Amy : I'm okay. The rain and wind could've been worse. Everything's fine.
大丈夫だよ。雨と風がひどくなるかと思ったけど、大したことはなかったよ。
Ken and Leo started speaking at the same time.
- Ken : Oh, sorry. Go ahead. It wasn't anything important.
あっ、ごめん。何？(僕の話は)大したこと(は)ないから。 - Leo : Okay. But I forgot what I wanted to say...hahaha.
そっか。でも、何を話そうか忘れちゃった…ははは。
Meg has a lot of work to do. She has to finish some of it by tomorrow.
- Amy : So, are you finally going to take care of those piled up bills? And if so, do you need a hand?
ついに(orとうとう)溜まった請求書を片付けるんだ？手伝おうか？ - Meg : It would be nice if you could check for any mistakes. It's going to be tough to go through all of them.
ミス(or間違い)がないか、確認してもらえると助かるな。全部やるのは大変だけど。 - Amy : OK. That's going to be a piece of cake for me ! But you need to buy me coffee after we're done!
了解。大したこと(は)ないよ！でも、終わったら、コーヒーをおごって！
Ken and Leo are in the amusement park.
- Ken : Hey, are you really going to go on this ride?
ねぇ、本当にこれ乗るの？ - Leo : Yes, why not? Since we're already here, let's do it!
うん、乗ろうよ？せっかく来たんだから、行こう！ - Ken : I'm just not a big fan of roller coasters. They're too scary.
ジェットコースターって、あんまり好きじゃない。怖いじゃん。 - Leo : Come on. It will be fun! It's nothing compared to Magic Mountain.
乗ろうよ。楽しいって！マジックマウンテンと比べたら、大したことはないよ。