To Stretch Something
transitive verb, godan verb
If 伸びる is "to be stretched" then 伸ばす is to stretch something, where you're doing the stretching. You know this because you took a bus and you stretched it out to be an even longer bus.
The reading comes from 伸びる, so if you know that you should know this.
子供がいたずらでゴムを伸ばしすぎて使い物にならなくなりました。
My kid mischievously overstretched the elastic and made it unusable.
祖母は死ぬ間際、私の顔に手を伸ばし、「愛してるよ」と言ってくれました。
My grandmother reached her hands up to my face and told me "I love you," right before she passed away.