Why are songs grayed out on Spotify?

Spotify's help center names two settings that gray a song out and make Spotify skip it: explicit content is switched off and the song is tagged E, or you hid the song in that playlist. When neither applies, the song may no longer be available where you are.

Explicit content is switched off

When explicit content is off, anything with an explicit tag is grayed out, can't be played, and is skipped. Look for the E next to the title. To switch it back on: profile picture, Settings and privacy, then Allow Explicit Content under Content and display on mobile; profile picture, Settings, then Allow playback of explicit-rated content on desktop. On a family plan, the plan manager may have blocked it for you. Details in What does the E mean on Spotify?

Source: Explicit content settings.

You hid the song in a playlist

You can hide songs in playlists created by Spotify or by someone else. A hidden song is grayed out and skipped in that playlist only, and still plays everywhere else. To show it again, tap or click the hide icon next to the song once more. In your own playlists Spotify offers no hiding: you remove the song instead.

Source: How to hide and unhide songs.

The song is no longer available where you are

Spotify's catalog varies over time and between countries, and depends on permissions from rights-holders. A song you saved can stop being playable in your country, and Spotify says missing content may reappear later.

Source: Missing music or podcasts.

Grayed-out songs in a Playlistation playlist

A Playlistation playlist is a playlist you own, so Spotify lets you remove a song from it rather than hide it. The next update puts a song back if it still matches the filters, because the playlist always holds exactly the liked songs that match (Edits made in Spotify). To keep a song out, change the filters, or unlike the song.

Playlistation does not check whether a song is playable in your country: a grayed-out liked song that matches the filters goes into the playlist, and Spotify decides whether you can play it there. To leave explicit songs out, add the Explicit content filter set to Clean only.