Since the responses you got don't actually go into the relevence of the Kate Moss reference, I'm going to copy/paste a response I gave to a similar question in another thread.

So for further context, during this trial 1 of the 14 cases of assault being discussed has to do with an incident involving Depp, Heard, & Heard's sister standing at the top of a flight of stairs. Heard claims that Depp made a motion as if to push Heard's sister down the stairs and so Heard therefore proceeded to attack Depp in defense of her sister. During her elaboration here she states "in my head [I] instantly think of Kate Moss and the stairs" which refers to a claim Heard had made prior in the UK trial that she had heard rumours that Depp had pushed Moss down the stairs during their relationship. A claim that was completely unsubstantiated and unfounded.

Now prior to the start of this case, from my understanding the judge barred calling past exes as witnesses since past relationships are moot to the current arguement. That is unless a point comes up to make them relevant, which is exactly what Heard did. This has opened the door for one definite outcome and one possible outcome:

The Definite: Depp's team can call Kate Moss to the stand to refute the rumour that Heard claimed - damaging Heard's credibility - as well as get Moss to testify to the character of Depp, who she has remained positive towards.

The Possible: This opens up the potential for any past relationship to be brought into evidence, which is also bad for Heard. This is because Depp's team could bring in Depp's ex Winona Ryder, who has been a staunch defender of Depp throughout the years, as well as bring into evidence Heard's arrest for misdemeanor domestic violence against her ex Tasya van Ree.

So by Amber mentioning Kate Moss and giving context that ties her to the current arguement, Heard has just undone the judge's barring by establishing relevance. Hence the celebratory fistpump and smiles on Depp's side.

Hope this clears up your confusion.