statusBarToolStripMenuItem.Checked = Not statusBarToolStripMenuItem.Checked
statusStrip1.Visible = statusBarToolStripMenuItem.Checked
ToolStripMenuItem.Checked doesn't mean anything by itself. The class has the Checked property, but you can only use properties of an instance of the class, not the class itself.
There may be several instances of a class, that's why instances have a name that is similar to, but different from the class name. You might have the instances statusBarToolStripMenuItem1 and statusBarToolStripMenuItem2. When using one of these instances, you would refer to it by its instance name, not the class name, which is ToolStripMenuItem. That way it is understandable, which instance you are using.