V_i is the velocity of the object at that instant or instantaneous velocity.
lim t to 0 states that the time we are observing the velocity is limited, and is now so short, that it is approaching 0.000000 sec
((triangled)/(trianglet)) informs us that the velocity is a function of the distance divided by the time, and since we are considering velocity here the direction must also be defined.
The triangle's tell us the function refers to the change in something, in this case the change in distance divided by the change in time.
As you might suspect, since the trianglet observing time is so short, the triangled distance travelled will also be short compared to the overall distance travelled. As a result, we can be sure that the calculation of the instantaneous velocity will be correct for the moving object, which may be either accelerating, decelerating, or moving at a constant velocity.
Here is more:
https://www.brightstorm.com/science/physics/linear-and-projectile-motion/physics-instantaneous-velocity/
