The Tesseract, to describe the real thing:
In geometry, the tesseract is the four-dimensional analogue of the cube; the tesseract is to the cube as the cube is to the square. Just as the surface of the cube consists of six square faces, the hypersurface of the tesseract consists of eight cubical cells. The tesseract is one of the six convex regular 4-polytopes.
This for us to look at from our 3d perspective would be like a 2d creature watching a 3d sphere fall through their plane of existence. Time itself is a human construct, but the change of an object is certainly universal.
It would look like nothing, then become a single point which then grows in radius to the final diametre of the sphere, to then finally reducing back down to a point and then nothingness, as such in this animation:
In the movie 5th dimensional beings have created a 3 dimensional "room/thing" where the 4th dimension (time) is physically represented. As described in this youtube.
To take from the script: (warning pdf)
TARS (over radio) Somewhere. In their fifth dimension. They saved us
...
COOPER (frustrated) Who’s ’They’? And why would they help us?
TARS (over radio) I don’t know, but they constructed this three-dimensional space inside their five-dimensional reality to allow
you to understand it ...
COOPER It isn’t working -!
TARS (over radio)
Yes, it is. You’ve seen that time is represented here as a physical
dimension - you even worked out that you can exert a force across
spacetime -
So the Tesseract isn't a time machine as much as it's a "Time Window" of sorts. Small data/Messages can be sent through the device via force, but no physical object can traverse.
This is similarly analogous to someone stuck in a sound proof glass house, you can't directly physically interact with the outside world, but you can go around the house to different windows and breathe on the glass and draw a message of some kind and hope the outside world somewhere understands.
