It's a adverb which is used before a verb and it means it will be like this automatically under some conditions,kinda like "then"
不斗争就不能前进(if ou didn't work hard, you would never get improve)
只要认真学习,就能取得好成绩(If you work hard, you will get good academic performance)
要是有房子就好了(If only we owned a house)