Any kind of danger that a person may feel. Following is the translation of the verse in question:

[This is the sharī‘ah of God. If you want to remain steadfast on it,] be watchful over your prayers, [624] especially the one which comes in the middle [625] [of the morning and evening prayers when it is not easy for you to take out time from your involvements], and stand before God devoutly [leaving aside everything].Then if you fear any danger, pray on foot or while riding, in whichever way you like, but when circumstances become secure, remember [626] God in the very manner [627] He has taught you, which you knew not.[628]

The ‘illat‘ in this case is Usar (undue hardhip, undue difficulty), so whenever and wherever one feels occupied in such a sitiuation, one can follow this directive.

As a principle, the relaxation in case of ibadah is based on the principle of usar (‘ghair mamoli mushaqqat‘) and for other prohibitions (‘mohramat‘), the relaxation is based on ‘iztirar‘ (becoming impossible to do or when it becomes a matter of life and death).

Please see for further details:

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For comments of Ghamidi Sahab to understand this specific issue from an epistemological stand-point, please refer to the video below from 1:54:08 to 1:59:41

To understand the matter in further detail, how ‘illat‘ is extracted from a Quranic directive, whose presence or absence determines that directive’s validity in other similar situations, please refer to the video below from 1:21 onward (specifically from 16:25 to 19:34)