Anti-aliasing defines how objects will be displayed within pixels. Text and objects should have anti-aliassing method set to "Smooth" (which is the default in Photoshop).
Objects with Anti-aliasing set to "none" may appear pixelated or rough when printed (even at high resolution).
Example of Anti-Aliasing (Zoomed in):
Anti-aliasing settings for text can be found in Photoshop in the expanded character palette, next to the "aa" icon.