"Pinned down" is a common military term for a unit that is currently being suppressed
by enemy fire. A suppressed unit has lost its ability to move, lost all or most of its ability to return fire, and lost much of its ability to gather real-time intelligence on the enemy position, as it is concerned with staying under cover.
Suppression is a key tactical goal utilized in most small unit tactics and will afford the attacking (or suppressing) unit with greater security of movement, intelligence, and tactical flexibility. Often, a suppressed unit will lose organizational efficiency and morale if kept in that state for an extended period of time.