Opposite action is a skill used in therapy to help manage overwhelming emotions in short term. This skill is used to help reduce intensity of overwhelming emotions (preventing harmful/ negative behavior).
When your emotions are pushing you towards negative behaviors, you need to take an action opposite to it.
For Example: if feeling sadness/ hopelessness, try to recognise these emotions might tell you to "isolate yourself, there is no point in trying" etc.
Now what you need to do is to take exact opposite action like trying to socialize with others, push yourself to go out, talk to people, to move your body and change your environment.
This way by taking opposite action, the intensity of that overwhelming emotion lessens and you start to feel better.