Pool of Reflection
From Diablo Wiki
The Pool of Reflection (generally called "PoR" or "Pool") is a yellow-colored pool that grants a 25% buff to experience gain that lasts for some set amount of experience, or until a character dies. This feature was added to Diablo III in version 2.0.1 in February 2014, and is also present in Reaper of Souls.
Pools replace some small percent of health pools (about every 10th pool) and grant a bonus to experience gain, but no healing bonus at all.
The bonus from the Pool does not get added and multiplied in with other experience bonuses, but instead works to modify the gross experience gained by .25. For example, if the "Bonus Pool Remaining" said 20m, the effect would add 20m exp to the next 80m exp gained.
Each Pool bonus adds 25% bonus experience on top of all other exp gained for some discrete amount of EXP. The bonus can stack up to ten times, potentially lasting about 4 levels, though the amount varies due to the differing amounts of exp required for each level.
 Bonus Experience Amount
Each Pool boosts the EXP gain for about 2/5 of a level up, or approximately 4 bars of the 10 in the belt interface display. When a Pool bonus is in effect a dot appears on the exp bar showing how much further the bonus will last.
A player can stack up to 10 bonuses from Pools, which is enough to last for about four level ups, though the amount varies due to differing exp required for level progression. Since higher level characters require much more experience to advance a level, a single Pool of Reflection can spread its bonus for tens of millions of experience for a character on an account with high Paragon Levels.
As a result high level characters who are not dying (such as in Hardcore) wind up with essentially a permanent Pool bonus, since it's virtually impossible to gain that much experience without encountering more Pools in the process. At the same time, a Pool used by a low level character will last a very short time, sometimes a minute or less if the character is killing quickly and using other +experience gear.
 Tooltip Information
The tooltip is confusing, though the little dot on the experience bar is clear enough for a visual indicator.
The displayed text is oddly-presented. The bonus number refers to how much extra experience on top of the normal amount the pool is providing. To calculate how much experience a character can gain before exhausing the Pool bonus, multiply the "Bonus Pool Remaining" number by 5.
Explanation: As you see in the screenshot above, this character (from the Reaper of Souls beta) has just started gaining towards Paragon 134, and requires around 200m for the next level. The Pool bonus is 25% on top of other exp gained, and it says 32m for the pool. So multiply 32 x 4 = 128m + 32m (pool bonus) = 160m. Which is right about 8 bars along the way to 10 bars / 200m experience.
Yes, a bit confusing and weird in display. Fortunately the dot on the experience bar is simple and visual.
 Sharing in Multiplayer
Pools of Reflection share their bonus equally to all players in the party, providing they are close enough to the pool when it is activated to partake in the bonus. The distance, according to Blizzard, is around 100 yards, which is a bit more than the visible screen (varies by a player's resolution settings).
During the initial testing on the PTR the Pools had a much smaller radius of shared effect, and players were required to gather very close to the pool in order to gain the bonus. Blizzard addressed this issue in patches and by the time D3v2 went live the radius of sharing was enlarged to work just like the Shrine bonuses.
Helpful players sometimes call out Pools in the clan chat or to their friends, then wait a moment for others to join and share in the pool benefit when activated.