It's often used to describe a feeling of nervousness, anxiety, or emotional distress that makes it hard to talk clearly.
For example:
* "I had a frog in my throat during the presentation, and I couldn't get my words out right."
* "I always get a frog in my throat when I have to talk to my boss."
The origin of this idiom is uncertain, but it's likely related to the feeling of constriction in the throat that can be caused by things like:
* Nervousness: When you're anxious, your muscles can tense up, including those in your throat.
* Dryness: A dry throat can also make it feel like there's a lump there.
* Physical irritation: A sore throat or other throat irritation can also cause this sensation.