What you are asking about is one of many "If" sentence structures in Japanese. Firstly, it's not actually ければ. It's え-stem of a verb + ば. Verbs with conjugation exceptions include: 来る、する, ない and the special るverbs than don't have ～らない endings in their simple negative form like 見る、食べる which become 見ない、食べない The most common way you'll see this grammar is in the form しなければならない, which means "I have to do it" but more directly translates to "If I don't do it, it won't do" or something bad will happen if I don't do it.

しなければ is translated as "must do." It's usually used in a conditional sentence, like 宿題をしなければ、遊べない (I must do my homework or I can't play/If I don't do my homework, I can't play). If you remember that "ば" is used to talk about something conditional, you could form "if..., then..." type of sentences. This can be used for verbs, adjectives, and nouns. Verb: ご飯を食べなければ、お菓子は食べれない。 Adjective: 寒くなければ、散歩に行こう。 Noun: 子供でなければ、この公園では遊べない。 I wish I could explain it better, but I'm not that good at explaining Japanese grammar in depth:/ Hope this helps!