If you watch videos or listen to audio online, you may see a term like "buffering" or "buffer full." The most common use of this term relates to streaming media over the Internet, but the term could apply to a more obscure function on your PC.
When you access streaming media like online videos and Internet radio, your computer usually "buffers" the data it's going to need soon. That means it downloads data ahead of what you're currently viewing or listening to, so it's available right when you need it. Buffering also reduces the possibility of pauses during the media's playback. If the buffer for the media is full, then your computer has downloaded everything the website permits it to download, though your computer will continue downloading data as you progress in the media clip.
The speed of your Internet connection is usually the limit to how quickly you can buffer data. If your connection is fast enough, you may be unaware that your computer is buffering the media content because playback is uninterrupted, but if you have a slower connection, the media may need to pause occasionally to buffer additional data, and the player should notify you that it's buffering. In rare cases you may experience delays in playback because the website can't deliver the data quickly enough, possibly because too many users are on the site. If the media stops frequently to buffer, you can pause it to let your computer buffer more data before you reach the playback point beyond which you don't yet have data.
Media buffering is used on many websites that host video content, like YouTube, Netflix Watch Instantly and Amazon Instant Video, as well as Internet radio stations like Pandora, Slacker Radio and Live365. Often you'll see an indicator, usually in the playback progress bar, for how much of the content has buffered, especially with videos. If the buffer gets full, the indicator stops moving or may even say that the buffer is full. Some sites don't give you buffering limits, meaning that you can buffer the entire media file.
"Buffering" also can refer to memory buffering for software on your computer. This process is independent of your Internet connection. These buffers are reserved locations in your PC's RAM (random access memory) where your computer temporarily stores data, frequently for files that the computer is accessing. This allows the computer to access and manipulate that data much more quickly than using it directly off of your hard drive, so your programs work more efficiently. If the buffer has received so much data that it's used up its allocated space, then the buffer is full.
Your computer could actually send more data to a full buffer in the RAM, and the data will have to go somewhere else since there's no room for it in the buffer. This is called "buffer overflow," and it's an error that could cause instability on your computer, though there's no prevention available to an average computer user other than using software from trustworthy developers.