Serious
Terrible, Very, Difficult, Hard, Hectic
adverb, な adjective
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.
大変な問題
a serious problem
大変な作業
an arduous task
大変な事
a difficult matter, a serious matter
大変失礼
very rude
大変重要
incredibly important
大変お世話になる
to be much obliged to someone for their help
かなり大変
considerably difficult, considerably serious
中々大変
rather difficult, rather serious
とても大変
very difficult, very serious
えー、めっちゃ大変だったんだね！
Oh, it was really tough, huh?
皆様には、大変心配をおかけいたしました。
I apologize for causing you all so much concern.
日本語をマスターするのは大変ですが、がんばってくださいね。
Mastering Japanese is difficult, but do your best!