When the game begins, you awaken in a kind of sleeping pod. The pod opens and you automatically get out of it and stand up. After a few seconds, a computer voice starts talking to you. She mentions that you have been kept in detention in a relaxation cell, but now your "specimen" has been processed and you are ready to begin the test. Then her voice gets really messed up, playing at the wrong speed and speaking in Spanish for a moment.
Clearly, things are kind of weird here, and I'm sure you have plenty of questions. What is this test? What does she mean by 'specimen?' Why did her voice get all messed up? And what's with that crazy elevator music playing on the radio? Why not start playing the game to find out?
After a minute has passed, the computer will create a portal to let you out of the relaxation cell. You might notice that you can see your person--a woman in a orange jumpsuit with jump springs on her legs--through this portal. You might realize now what a portal is: a special warping spot that creates a path between the orange one and the blue one. There is no difference between the orange one and the blue one; they are just the two ends of the portal.
Anyway, let's move on. Exit through the portal and continue on through an open door. The door closes behind you and a grey cube falls from a tube in the ceiling. There is a big red button in the room. Go over to the cube, pick it up by pressing the E key on your keyboard (or whatever you set the picking-stuff-up key to be in your key mapping settings), and walk over to the red button. Drop the cube on it, and the next door will open. The computer will praise you for your good work, and tell you to continue on, but will explain about the particle field that sits between you and the elevator to the next level. It destroys any items that you try to bring through it, like the weighted storage cube that you used to push the red button.
When you're ready, go through the particle field and enter the elevator to go to the next level.