Most calculator screens can display at most 10 digits. Therefore, for numbers greater than 1,000,000,000, calculators incorporate the E symbol, which comes from scientific notation. This allows calculators to indicate very large values.
E represents 10 to a given exponent. For example, 5.23 E 4 means 5.23 x 10^4. This is a short-hand of writing 52,300.
By using E, the calculator can display both very large numbers or very small numbers. For example, 5.1 E -4 means 5.1 x 10^ -4, which equals 0.00051. When there are many zeroes, like for 0.0000000000000000001, which the calculator cannot display, the E helps shorten this to a readable form.
E is different than the natural number, e, which mathematicians use to calculate compound interest, and the -e- symbol, which stands for error. More often than not, you will find E after calculating a very large number like 5^20 or a very small number like (0.01)^12.