The "repeat marker" enables you to repeat the tune in the playlist from where the repeat marker is positioned. No, it's not a "time marker", it's a "repeat marker"!
You create it by right double-clicking on the timeline - it looks like a small arrow pointing to the left - and you move it by right-clicking (keep it pressed) and dragging. Now, the hard part ... How do you get rid of it? By dragging it all the way to the left.
When you place it past the end of the song, it's a bit like you're making the song longer and the rendering of the tune into wav or mp3 files will go past the end of the song to the marker. This will make sure any decaying sounds are captured in the export file.
I am kinda showing all that in the tutorial video below.
In FL Studio 10, the "repeat marker" is no more as it has been replaced by the "time marker" (Playlist Options->Time markers->Place song loop). You can move the time marker with the left mouse button and delete it with a right mouse click followed by delete.