To the right of the Use Center flyout on the Main toolbar are the Restrict to ... buttons, also called the Axis Constraint buttons. These let you specify one or two axes about or along which the transform takes place.
Note: It's generally easier to use the Transform gizmo than these buttons; see Using the Transform Gizmo. But it is helpful to understand the concepts explained below.
Only one of the four Axis Constraint buttons is active at a time. When a button is turned on, transforms are constrained to the axis it specifies. For example, if you turn on the Restrict to X button, you can rotate an object only about the X axis of the current transform coordinate system.
The axis or axes to which youÆre constrained are highlighted in red on the axis tripod icon in viewports, or in yellow on the Transform gizmo.
The first three buttons constrain the transform to a single axis. The fourth button is a flyout, the Restrict to Plane flyout, that specifies double-axis combinations.
For example, if the Restrict to YZ Plane button is active, you can move an object only along the YZ plane, you can scale it only along the Y and Z axes, and you can rotate it about the Y and Z axes (or a combination of the two) but not about the X axis.
Note: Normally axis constraints apply when using Snap. You can override this by turning off Use Axis Constraints in the Snap Options Settings.
As an alternative to clicking the buttons on the Main toolbar, you can use the four function keys F5 through F8 to select the four axis constraint buttons. F5 activates the X button, F6 the Y, F7 the Z, and F8 activates the double-axis flyout. Press F8 repeatedly to cycle through the three double-axis buttons.