What do we mean by Byte array?

A byte is 8 bits (binary data).

A byte array is an array of bytes (tautology FTW!).

You could use a byte array to store a collection of binary data, for example, the contents of a file. The downside to this is that the entire file contents must be loaded into memory.

For large amounts of binary data, it would be better to use a streaming data type if your language supports it.

I assume you know what a byte is. A byte array is simply an area of memory containing a group of contiguous (side by side) bytes, such that it makes sense to talk about them in order: the first byte, the second byte etc..

Just as bytes can encode different types and ranges of data (numbers from 0 to 255, numbers from -128 to 127, single characters using ASCII e.g. 'a' or '%', CPU op-codes), each byte in a byte array may be any of these things, or contribute to some multi-byte values such as numbers with larger range (e.g. 16-bit unsigned int from 0..65535), international character sets, textual strings ("hello"), or part/all of a compiled computer programs.

The crucial thing about a byte array is that it gives indexed (fast), precise, raw access to each 8-bit value being stored in that part of memory, and you can operate on those bytes to control every single bit. The bad thing is the computer just treats every entry as an independent 8-bit number - which may be what your program is dealing with, or you may prefer some powerful data-type such as a string that keeps track of its own length and grows as necessary, or a floating point number that lets you store say 3.14 without thinking about the bit-wise representation. As a data type, it is inefficient to insert or remove data near the start of a long array, as all the subsequent elements need to be shuffled to make or fill the gap created/required.