A PCB Stencil is a sheet of stainless steel with laser-cut openings used to place solder paste on a PCB board for surface mount component placement. 

The PCB stencil is used to deposit solder paste on designated places on a bare PCB board so that components can be placed and perfectly aligned on the board. The working life of a stencil is calculated by the times of using, around one hundred thousand times.  And the stencil cost is always one-time charge, unless you change the design of the PCB for the new production.