That's called aliasing, and it can be an expensive problem to fix, performance wise.
To address the situation in your game, turn on antialiasing if you can find it in your settings for the game. The higher the antialiasing in the game, the better it will look and the higher the performance penalty the graphics card will take from doing the extra work. There is definitely a point of diminished returns with antialiasing, so getting ham handed and just turning it to max isn't always advised. Try the lowest settings first, play the game a bit, and see what is tolerable. Once you get to a certain point, the aliasing will be diminished enough you will mostly forget it's there, and lowering performance further to deal with it is unnecessary. The only time I recommend high AA is when playing older titles, where your GPU can do full AA and still maintain high enough performance you don't see the penalty.