The compendium content includes the full pages of content that you can read through - in essence all of the content of the physical book. Think of it like an online browsable version of the Player's Handbook.It does NOT include digital assets that can be used with the character creator.
For example, if you purchase, PHB Compendium Content, you have the full Player's Handbook that you can read, reference and search through using D&D Beyond. However, you would not unlock the ability to use the additional subraces, subclasses, spells, feats etc for use in the character creator.
If you should later decide that you would like the digital assets as well, you can upgrade to the full Player's Handbook by paying the difference.