Terrible, Very, Difficult, Hard, Hectic
Adverb and な adjective
たいへん
日本語を習とくするのは大変ですが、がんばってくださいね。
Acquiring Japanese is difficult, but do your best!
大変丈夫な子供が生まれました。
They had a very healthy child.
災害時は、電話回線もネット回線も混み合って、繋がりづらくなるので、ブログの更新が大変です。
When there's a disaster, the phone and Internet circuits are overloaded and it gets hard to connect to them, so it's difficult to upload new blog posts.
A big change is a really serious thing. And people don't like change, either, making it a terrible event.
Also, in more recent history this has come to mean very as well.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.