.srt files are sidecar subtitle files. They can be used to add subtitles to .mp4 files.

Put the .srt right next to the corresponding .mp4, with the same name.

Open the .mp4 with VLC.¹⁾

If the subtitles did not automatically appear, this is how you can add them manually:

Right click into the video > Subtitles > Add Subtitle File

Navigate to the folder with the .srt and open the file.

VLC will give feedback in the upper right corner, that the Subtitle track has been added!