It's a more informal and poetic way to describe a stretch of time where the sun is shining brightly and there are no clouds in the sky. It typically implies a few days or even a week of pleasant, warm weather.
For example:
* "We enjoyed a sunny spell during our vacation."
* "The forecast predicts a sunny spell this weekend."
Essentially, it's a brief burst of sunshine that breaks up a potentially gloomy or overcast period.