The difference is as intuitive as it gets. Freestanding tents have a pole structure that keeps the tent erect even when it’s not fixed to the ground with stakes and guy ropes. Non-freestanding tents require you to stake them to the ground, or they collapse.
Freestanding tents are easy to move around the campsite without breaking them down. This allows you to move the tent quickly, for instance, if it starts to rain and you figure out water is gathering under the tent.
Non-freestanding tents are not as easy to move but are often lighter and cheaper than freestanding models. Which is the best 2 person tent is ultimately down to you, based on your needs and budget.
If you want a lightweight freestanding tent, pick a model with aluminum poles. These tents might be more expensive, but the poles are very light and sturdier than fiberglass.