“No matter what happens, I'm there for you.”
Your best friend has been offered her dream job. But if she takes the job, she will have to move to a new city and break up with her boyfriend. She is really upset about the difficult choice she has to make. You reassure her by saying this.
No matter what happens, I'm there for you.
This phrase is used when you want to say that some conditions don't matter. The result, or your advice, will be the same in any of the conditions. For example:
No matter what she says, don't listen to her.
In this examply, your advice ("dont listen to her") is the same, regardless of what the person says.
You can use a dependent clause, like "no matter what she says", or you can use the word "happens", like in the example at top.
You can also use just the phrase "no matter what" to mean in any condition:
No matter what, I'll always love you.
This means to support someone. When you say that you will "be there for" someone, it means that you wil be available to listen to them and support them emotionally. In the example at the top, you are telling your friend that you will support her decision whether she stays or goes.