When a force acts over an area, it is called "pressure". A higher pressure pushes (accelerates) fluid toward lower pressure. So any change in a fluid's speed must be matched by a change in pressure (force). Bernoulli saw that while the fluid moved more quickly in the smaller part of the tube, the pressure became less. Please note that this refers to changes in speed and pressure along a single path of flow and does not apply to two different flows at different speeds.