This is a remote control for Nemp - the player you are currently listening too. Depending on several settings, you are allowed to do more or less with the playlist.
On the player page, you may have access to common controls like 'play/pause', 'next/previous title', volume control or progress.
It is also possible that you can download the current song to your device.
On the playlist page you can see all tracks that are currently in the playlist. In Party-Mode, you can usually just 'vote' for a file.
Voting for a file means 'I like this file, and I want to hear it soon'. On the playlist page you can see the amount of votes for every file. If you see no votes, then nobody voted for this file yet. You can vote for a file more than once, but only once per minute.
On a more private party, where the DJ trusts you, you may have more options to control the playlist, like 'play now', moving files up/down or deleting files from the playlist
On the library page you can browse in the available medialibrary. You can browse by artist, album or genre, or just search for something you like. The search is fault-tolerant. So, if you have some typos, the system may find something matching anyway.
If you vote for a file in your search results, it will be added to the playlist. If it is already in the playlist, it will get another vote from you.
Again, on a private party, you may have more options, e.g. 'Add the file to the playlist' or 'Add it after the current title'.