If you can "bear" something, it means that you can "stand" it or survive it. Something that's unbearable is too bad or difficult to accept. 

"Unbearable" pain is really strong pain that makes you cry, scream, or feel like you're going to die.

Other things that are "unbearable" include:

• unbearable heat
• unbearable suffering
• unbearable tension (in a stressful environment)
• unbearable conditions