When this happens, Soundtrap automatically freezes your tracks to save memory. Freezing in the studio is basically the same as "rendering" your tracks for playback. It helps improve 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 real-time as it is being played.