Stop out happens when you open big trade and you don't have enough margin in your account to maintain it active.
In other words, your position was already in a losing state, and the Stop Out mechanism simply closes your unprofitable position gradually so your account doesn't burn out.
Your margin level dropped too low - specifically below 100%, which triggers a Stop out. This means when your equity gets too close to or lower than the margin used, trades start automatically closing to free up margin.
Only applicable to users specifically asking about Stop out. Not applicable to any other situation.