Making OpenGL calls has a performance cost. It is therefore desirable to reduce the amount of calls.
OpenGL has global state. The state is modified by enabling/disabling blending/culling/alpha testing/etc, specifying the blend function, changing the cull face, etc.
A state leak happens when a method leaves OpenGL in a different state after being called. This is generally undesirable as it often breaks expectations about the OpenGL state in other methods.
Modifying the OpenGL state requires OpenGL calls. Minecraft is not particularly optimized for reducing the amount of OpenGL calls, but makes an attempt using GlStateManager.