Back in the good old days, when you started watching a YouTube video, the entire thing would download (or “buffer”) in the background, thus ensuring a relatively smooth playback experience.

If you had a slow connection, you could simply pause the video until you saw the “buffer bar” complete its journey from left to right. But thanks to YouTube’s switch to a new protocol (Dynamic Adaptive Streaming over HTTP, or DASH, if you’re interested), that trick no longer works.

Now, when you play a video, YouTube buffers only a small amount. And that can lead to stuttering, frequently interrupted playback.

Full article here:
Force YouTube to buffer your entire video (PCWorld)
Related:
How to Preload and Buffer Entire YouTube Videos By Disabling DASH Playback (TechNorms)
Go back to YouTube’s old buffering to skip without rebuffering (gHacks)
Preload Entire YouTube Videos By Disabling Dash Playback (Lifehacker)