Let's start by naming the parts of the interface...
The main view. Click and drag with your mouse or touch and drag to look around.
Mouse wheel or two finger drag to zoom.
Performance graphs. Similarly click and drag to set the current time or zoom.
Your username (with a link to log out)
The clipboard copies a link for the current view to your clipboard
The box icon enters full screen mode.
Timestamp shows the time within the recording.
The control bar has controls for run/stop (the spacebar will also start and stop the playback), and 'x1 x2 x5 x20' control the playback speed as multiples of 'real' time.
Set Start is a control that only appears if you're logged in and this is one of your recordings. Many recordings start with lots of getting out the the marina, hoisting sails etc. so you can put the recording at a sensible start time (and camera view) then press "Set Start" and the recording will start from this point next time you play it.
Various arrows, lines etc. are drawn on the main view attached to the graphic of your yacht...
The green/red stripe shows the shape made by the mast travelling through the air and is therefore a record of heel (and to a lesser extent pitch) over time. When it's green, you have been travelling at 100% (or more) of the highest recorded speed-over-ground during the last minute. It becomes gradually redder to indicate when this speed has dropped. This is therefore your primary notification that something slow is happening and you should probably find out why.
The black/white stripe shows the true wind you were experiencing as you passed over that piece of water and is therefore a record of wind speed and direction. Similarly to the speed stripe, this is white when wind is at 100% of the peak wind speed in the last minute, becoming darker to represent lulls. Often you will see these dark lulls lining up with red patches in the speed stripe, so you know that that particular slowness was wind rather than something you did.
The long yellow line is drawn perpendicular to the true wind direction at the yacht and thus represents the "equally windward" line used when visualising tactics. The numbers are true wind speed in knots, and absolute wind direction in true degrees.
The white arrow represents the speed and course over ground in true degrees. The numbers are speed in knots and the wind direction relative to the yacht's course ie. the pointing angle and not the yacht's heading.
The green arrow shows VMG and is therefore drawn at a right angle to the "equally windward" line. The number is vmg in knots. Note as well a much finer gray line that shows the wind angle experienced across the deck, and a yellow 'windex' at the top of the mast.
Finally, the light grey line just shows the course the yacht took during the recording.
The two performance graphs share a common time axis and conception of "now". So if you drag one, the other will move; and if you zoom one, the other will zoom to the same extent. The red line in the centre denotes the current time (within the recording) as shown by the timestamp on the bottom right.
The left hand graph is dedicated to angles and shows the heading and course over ground, and true wind direction. Note that the difference between heading and cog should be the leeway angle, but is often merely the calibration error of the compass. Work to automatically correct this is 'on the list'.
On the right are velocities: speed over ground, vmg and true windspeed. Of note is how you can see the vmg temporarily peaking as the yacht enters a tack.
The clipboard copies a link to the current view to your clipboard. You can then paste it into an email, shared message or use it as a link in HTML. The clickable link for the picture at the top of this page was made this way (try it!).
Full screen sets the main view to occupy the entire screen and removes the graphs in the process. You can still look around, and the spacebar will start/stop the playback. Press 'esc' to return to normal operation.
One more thing... you can pan around the view by dragging with the right mouse button, or using two fingers on a trackpad. This only works when playback is stopped, however.