It essentially refers to something someone is naturally good at or has developed a strong ability in.
Here are some examples:
* "Public speaking is not my strong suit, but I'm a great writer."
* "Negotiation is her strong suit. She always gets the best deals."
* "Cooking is definitely his strong suit. His food is amazing!"
The idiom can also be used more generally to describe a person's strengths:
* "He's a great leader, but his strong suit is motivating his team."
* "Her strong suit is her ability to stay calm under pressure."