The term 'EV mode' can mean one of two things. First, the term 'EV' means 'Electric Vehicle', implying a car that can ride only on electric power, no gas. (Such as the discontinued GM EV-1, which needed to be plugged in at night.)
So when talking about the Prius, it means running in electric only mode. The two ways this relates is either the built-in 'stealth' mode, where you drive very carefully and slowly, accelerating like an old grandma, and it only uses the battery. But it is hard to stay in electric-only mode, because the car really wants to use the gas engine in most cases. It is possible to go up to 42 miles an hour this way, but good luck, about the fastest I can regularly go this way is 20 on level ground. If you hit the accelerator even a little too hard, the gas engine will kick in. And if the battery gets too low, the gas engine will kick in. If you try to go uphill at all, the gas engine will probably kick in. This is rarely called 'EV mode', because of:
The second method is to engage the Prius' native 'EV mode', where it specifically prefers the battery. In this mode, you can accelerate a little harder before the gas engine kicks in, and drive more normally. The catch is that the battery really doesn't last very long this way, and you can get at most about 2 miles on 'straight and level' ground. This 'EV mode' is engaged with a button on the dash on non-US models, but the button was removed from the US model. The only way to engage this on a US Prius is to add an aftermarket button. One way is by actually adding an 'EV button', but the more common way is to modify either the light stalk or the cruise control stalk so that they double as an 'EV mode button'. This way, you can accelerate a little harder than the above 'non-EV-mode-electric-mode', but if you floor it, the gas engine will still kick in (for safety reasons. If you floor it, you must really need the power.) And in this mode, you can only go up to 34 miles an hour before the gas engine kicks in. But, you can easily go up to 34. Maybe not uphill, but you can at least go uphill at SOME speed on only the electric motor. (And watch that battery meter drop like a stone.)