A trillion is 1,000,000,000,000, also known as 10 to the 12th power, or one million million. It’s such a large number it’s hard to get your head around it, so sometimes trillion just means “wow, a lot.”
A trillion is bigger than a million, more than a billion, it’s 1,000,000,000,000 (and even or 1,000,000,000,000,000,000 in some countries). (Yes, those are 18 zeroes in that last one.) Either way, this is such a large number that it's hard to imagine. Maybe that's why people often use this word in a looser sense, as in "I have a trillion reasons why I like this band!" That meaning is similar to gazillion, bazillion, and zillion, which aren't real numbers.