Throws (often interchangeably called ‘grabs’ depending on the animation) are techniques that grab the opponent. A closely related topic is the throw hitbox (a throwing move being a move with a throw hitbox for its attack hitbox).
The main point of throwing is to circumvent blocking. In fighting games the term ‘throw’ does not just refer to flinging the opponent; it can include grabbing the opponent in any way, which can be followed by a predetermined attack sequence, and so forth (though it should be noted as a matter of semantics that not all things that lock are thought of as throws).
In 2D fighting games, the startup of throws tends to be quick; in 3D fighting games, it tends to do a lot of damage.
Beginners are often confused by throws, because it doesn’t seem fair at first. The general trick to avoiding throws (other than where throw escaping is easy or you can hit the opponent before they can grab you) is to avoid being where the throw can grab (see grab range). Most throws in a 2D fighting game are on the ground, so even the slightest state of being in the air at the right time (anywhere from jumping to a slight hop caused by other means) will render these particular throw attempts unsuccessful.