Freezing in Soundtrap is basically the same as "rendering" your tracks for playback. It helps in improving performance to avoid overloading your device's memory usage.
If you're working on projects with multiple tracks or effects (like reverb) which can cause high demand for playback, there is a bigger chance your computer's memory will get overloaded. This is because the effects in your tracks are applied in realtime as it is being played.
When this happens, Soundtrap automatically freezes your tracks to save memory. Once it's done freezing the tracks, you'll notice how smooth Soundtrap runs at this point and now you're free to finish that masterpiece!