A colloquial contraction of 'というのは'. Used to introduce explanations, definitions, or topics, similar to 'as for' or 'that is to say'.
彼ってのは、本当に頼りになる友達だよ。
As for him, he's really a dependable friend.
専門用語ってのは、初心者には分かりにくいことが多い。
Technical terms, that is, are often hard for beginners to understand.
時間がないってのは言い訳にすぎない。
Saying you don't have time is just an excuse.
彼女が来ないってのは、本当なのか確認したほうがいい。
You should check whether it's true that she isn't coming.