electric fields - Work done in moving a charge

So, recently while I was studying electric fields and charges for a test, I came across the definition of electric potential as -

The amount of work done in moving a unit positive test charge from infinity to a point, without changing its kinetic energy, i.e, without acceleration against the force due to an electrostatic field.

But this contradicts to what the work-energy theorem says, that the work done equals the change in kinetic energy. What am I missing here?

Also, what exactly is electric potential? Why can we only define a potential for a conservative field?