Fantasy and its subgenres give you a bit more leeway in your science than, say, an action thriller. But just how technically accurate it needs to be will depend upon the type of fantasy you're writing.
Douglas Adams gets down right fanciful, with his Infinite Improbability Drive and Bistromathic Drive. Again, his amusingly logical linguistic tricks, make his technology seem almost, but not entirely, believable.
Fanciful science fiction works, in part, because the writer understands basic concepts of real science. It's the same with plausible science fiction. Understanding how real life lasers work can help you invent plausible laser weapons. *Laser Weapon article.
The world you create must be consistent to itself. I touched on this in an earlier post. You may want to sign up for author, Karina Fabian's, Worldbuilding class.
Consider what subgenre of fantasy you are writing. What makes sense? Even if you're writing a Tolkienesk fantasy where the highest technology are bows and catapults, you'll want to know how those contraptions work. How are swords and armor forged? Which metals should be used? That's not to say that you will include a three page scene featuring a blacksmith, but knowing about it will help you, the writer, understand the objects so familiar to your characters.
Lastly, pass your story to someone you can trust to give you a true reaction and see how much eye-rolling you get. You'll learn what aspects are not believable and hopefully which details are simply inaccurate.
Your nurse friend will mention how you messed up that scene about the syringe. The gun enthusiast in your critique group will chuckle as he reminds you that your character should have reloaded by now. Thank these people, no matter how dumb they make you feel. They'll help you grow wiser.