You can use Fade In and Fade Out for video and audio.
One example might be if you want to fade in and fade out a text title or if you want to fade in or fade out the sound on audio file.
The shortcuts are found at the Timeline level...
If you have a video audio clip, right click the clip, select Fade, and then decide which of the options that you want
Fade In Audio
Fade Out Audio
Fade In Video
Fade Out Video
With a particular selection, Opacity keyframes are placed on the clip's rubberband (orange line that runs horizontally across the duration of the clip). For a Fade In Video, you would expect one Opacity 0% keyframe at the beginning of the clip and second one 100% at 1 second from the beginning of the clip. For a Fade Out Video, you would expect one Opacity 100% keyframe @ 1 second from the end of the clip and another one 0% at the end of the clip. These keyframes are adjustable at the Timeline level.
You can also do these fades under Applied Effects Tab/Applied Effects Palette/Opacity Panel expanded. Same result clicking Fade In and Fade Out tabs there as with using the shortcuts at the Timeline level.
Please let me know if you are OK with the above information.
I clicked on those triangles but I didn't see any difference in the timeline or anywhere.
Premiere Elements 13/13.1 on Windows 7 64 bit...
Example of what you would be expected to see for a Fade In Video
After click on Fade In in the Opacity Panel expanded of the Applied Effects Palette under Applied Effects Tab
When I clicked Fade In, two Opacity keyframes were placed at the beginning of the Timeline rubberband...follow the rubberband (orange line) in the video clip (when the orange line is at the top of the frame that is 100% Opacity, when at the bottom, that is 0% Opacity)........the first Opacity keyframe was automatically set for 0% Opacity at the start of the Timeline clip. That is why you are see a black screen with the Timeline Indicator at the start of Timeline video clip....the second Opacity keyframe was automatically set for 100% at 1 second from the start of the Timeline clip.
Note in the screenshot at 1 second, the full video clip is displayed. Gradually from 0 to 1 second, the view went from no video to full video...Fade In.
You can move those Opacity keyframes (white dots) on the Timeline with the mouse cursor to alter the default result to achieve your 0 to 100% increase in Opacity (Fade In) at another location and have a different duration between 0 to 100% increase.
Please review and take another look at the effect. Then please let us know if you can get the Fade effects.