You create a ward against magical travel that protects up to 40,000 square feet of floor space to a height of 30 feet above the floor. For the duration, creatures can't teleport into the area or use portals, such as those created by the gate spell, to enter the area. The spell proofs the area against planar travel, and therefore prevents creatures from accessing the area by way of the Astral Plane, Ethereal Plane, Feywild, Shadowfell, or the Plane Shift spell.
The phrasing uses both phrases, "into" and "enter". If you teleport from one area to another, you are going into an area, and if you enter an area that would only block teleportation from outside.
As such, I'm not sure whether the spell could be used, say, to ward a battleground so that the enemy couldn't teleport around within said area easily, or used within a mage's private area to prevent an enemy teleporting around smashing things.
So, can I teleport within an area warded by Forbiddance?