A preamble is a signal used in network communications to synchronize transmission timing between two or more systems. In general, preamble is a synonym for "introduction."
The role of the preamble is to define a specific series of transmission criteria that is understood to mean "someone is about to transmit data". Proper timing is required to ensure that all systems are interpreting the start of the information transfer correctly. The actual preamble varies depending on the network communication technology in use.
In wireless transmissions, the radio preamble (also called a header) is a section of data at the head of a packet. The length of the preamble can affect the time it takes to transmit data by increasing the packet overhead. Packet overheard includes all the extra bytes of information that are stored in the packet header. When combined with the assembly and disassembly of packets, increased packet overhead affects throughput by reducing the transmission speed of raw data.
WiFiHowTo.org explains the difference between short preambles and long preambles.