What is E in floating point?

E notation

Most calculators and many computer programs present very large and very small results in scientific notation. Because superscripted exponents like 10⁷ cannot always be conveniently displayed, the letter E or e is often used to represent times ten raised to the power of (which would be written as "x 10^b") and is followed by the value of the exponent. Note that in this usage the character e is not related to the mathematical constant e or the exponential function e^x (a confusion that is less likely with capital E); and though it stands for exponent, the notation is usually referred to as (scientific) E notation or (scientific) e notation, rather than (scientific) exponential notation (though the latter also occurs). The use of this notation is not encouraged by publications.

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As for your second question:

can we use this approach in other data type or we can only use it in floating points?

See the C# spec:

Real literals [the type of numeric literals that are allowed an E in them] are used to write values of types float, double, and decimal.

However, you have to cast or suffix the literal appropriately when assigning to anyhting other than a Double, because any literal with an e or E in it is recognized as a Double in Visual Studio. I cannot find where this behavior is specified.

float f1 = 7E1;     // Compile error. Needs F suffix (7E1F)
decimal d1 = 8E2;   // Compile error. Needs M suffix (8E2M)
double d2 = 9E3;    // Works.

int overninethousand = (int)9E3 + 1; // Works

E+3 means the decimal place is moved 3 times to the right

1.7 -> 17.0 -> 170.0 -> 1700.0

In general, that's exponential/scientific notation...

1.7E+3 = 1.7 x 10^3 = 1700