Slow-Motion means "slow movement". So, this refers mainly to slow-motion recording with specialized photographic sensors. However, TikTok lets you make these types of recording easily from its application.
To start such a recording, open the TikTok content creation section.

Tap on the Speed button, located on the upper right side.

When this is done, a speed selector will appear above the shutter button.

Both 0.3x and 0.5x options correspond to the slow-motion mode.

Select one or the other, and then, tap the Circle button to record.

After that, all recorded videos will be slowed down by TikTok. You can combine videos in slow motion, with others in fast motion or even at normal speed. All you have to do is alternate the speed with the selector.
After this explanation on how to activate slow-motion in TikTok, we must make some clarifications. It is important to know that, although the application enables this function to be performed by any phone, the result may vary from one phone to another. In top tier devices, with photographic sensors ready to capture a higher frame rate, the final result is usually better.
If your phone supports slow-motion recording, we recommend that you capture the video from the native camera application for the best results. Later, you can use it in TikTok. Of course, you also have at your disposal many third-party applications that are capable of recording or applying this mode to any video file.