DistroKid's upload form includes a checkbox for "Loudness Normalization."
When you select this optional extra, DistroKid will automatically adjust the level and headroom of your audio to Spotify's recommended settings, -14dB integrated LUFS with -1dB true peak maximum. Your newly adjusted audio will be sent to all streaming services you've selected.
For more information about Spotify's recommendations, see the "Mastering Tips" section of this page from Spotify for Artists: https://artists.spotify.com/help/article/loudness-normalization
Streaming services recommend these or similar settings to reduce the likelihood of distortion, clipping & other problems that can arise during each streaming services' transcoding & playback processes. For information about Spotify's transcoding processes, for example, visit https://artists.spotify.com/help/article/audio-file-formats