Because the amount of progress that can be made in Clicker Heroes is infinite, numbers like your DPS, monster health and your Gold total keep getting longer and eventually won't fit onto your screen anymore. To prevent an unorganized screen full of numbers, the developers have implemented two different Big Number Notations.
Notation using Symbols
The first notation uses a set of symbols with the first symbol substituting for 3 digits and each symbol thereafter substituting for 3 more digits. For example, the game will substitute 156,368 for 156K (K being the first symbol) and 273,353,924 for 273M.
The developers have implemented a condition as to when the game switches to the next symbol. They had to make sure the game didn't omit too many digits, or it would become hard to compare things like hero prices to your current Gold amount. (If a specific hero costs 1900 to buy and you had 1100 Gold to buy it, it would be confusing to display both numbers as 1K). The condition is that there can only be 5 digits on the screen at one time. This means numbers like 25,458 (5 digits) will stay the same, but the number 125,458 (6 digits) will be replaced with 125K. When this notation is enabled, all numbers will between 3 and 5 digits.
List of symbols
After the game reaches 100,000*, it will automatically switch to scientific notation. See Units.
The second notation will always show the 4 most important digits, instead of 3-5 digits, and will display the number in the following form:
This means for example that 236,947 will be displayed as 2.369e5 and 583,413,581,235,138 will be displayed as 5.834e14. If you wanted to switch the Symbol notation to Scientific notation, an example would be 600K = 6.000e5
Scientific Notation automatically activates after the total number of digits reaches 69, but it can be manually activated by checking the 'always use scientific notation' button in the settings screen. Numbers below 6 digits will never be scientifically abbreviated.
The option to always use scientific notation was added in patch 0.13.