VarInt

A VarInt (variable length integer) is a compact encoding used throughout the Bitcoin protocol to represent an integer using as few bytes as possible, depending on the value being represented. VarInts are commonly used to encode the length of a following list or array of items, such as the number of transactions in a block or the number of inputs/outputs in a transaction, so that small values do not waste space on unnecessary leading zero bytes. A VarInt is encoded as follows:

Value range	Encoding
0 to 252	A single byte containing the value.
253 to 0xffff	The byte 0xfd, followed by the value as a 2-byte little-endian integer.
0x10000 to 0xffffffff	The byte 0xfe, followed by the value as a 4-byte little-endian integer.
0x100000000 to 0xffffffffffffffff	The byte 0xff, followed by the value as an 8-byte little-endian integer.
This encoding ensures that the vast majority of values seen in practice (such as small transaction or input/output counts) require only a single byte, while still allowing arbitrarily large values to be represented when needed.