Flow control refers to the need to prevent a sender of data from sending it so fast the receiver cannot take it all in.
In asynchronous serial lines, a variety of techniques are used.
With RS-232 interfaces, there are a pair of control lines dedicated to doing flow control in hardware - Request To Send, and Clear To Send. The sender activates RTS when it has data ready; the receive puts up Clear To Send as long as it's OK for the sender to send.
More commonly, serial lines also use software flow control, using in-band transmission of XOFF and XON characters: usually, but not nessarily, CTRL-S and CTRL-Q. (The formal ASCII specification does not reserve XON/XOFF characters.)
When the two instances are on a single machine (e.g. using a pipe to communicate), flow control is fairly simple to peform; the operating system monitors the amount of data in the stream, and blocks the sender when the backlog gets too large.
Over a network, the problem is a little more complicated; the protocol has to have a mechanism to allow the destination to inform the source how much buffering is available for incoming data, or to tell the source that the destination cannot accept any more data at this time.
In the TCP reliable byte stream protocol of the TCP/IP protocol family, a window at the sender is maintained for this purpose; the receiver on each side periodically updates the window stored at the other end.