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Consider the degrees of freedom your rectangle has. Assuming the sides are constrained to be horizontal or vertical:

1. The sides can change length.
2. The entire rectangle can move in the sketch plane.

You've constrained #1, leaving #2.

You have several options. You can drag one of the vertices onto the origin, which should create the coincident constraint, which will constrain the rectangle's movement. Often, dragging one thing onto another will implicitly create the constraint you want. You can also, say, use the dimension tool (shortcut is d) to specify the distance between the origin and a pair of sides. Each will constrain movement in a specific direction.

If you hope to understand the terminology that others speak of, its best to run through the basic courses of the learning center. Can you link youtube video for reference? basically michael is saying you need to dimension the location of the rectangle with respect to the origin of the design or in relation to other parts of the model if they already exist. basically so the program knows that your don't want the rectangle 100 feet way from where it currently in case something upstream of the that feature changes.

@gary_cummings - Remember when you start a new sketch, The origin is the only thing that is fixed in 3D space, and at least one piece of geometry needs to be 'anchored' to it, or your sketches will never be fully constrained. This 'anchoring' can be done with constraints, dimensions, or a combination of the two. I would also like to say, that all the above suggestions, seem valid as well.

@ gary_cummings Constraints are one of the more confusing things for beginners to get a handle on. You need to know what they are, and how they are used. Search "working with constraints" in the help section - or take the free beginner classes in the "learning center". I suggest you start a practice sketch and play with all of the various types of constraints (create lines, arcs, and points, then "constrain" them together) and do this over, and over. Get a feel for the different way, that each type works. While you are learning, you might want to check the "show constraints" box, in the sketch user interface (UI).

You must also learn about inferring constraints at the time of geometry creation ( this can cause trouble if you do it without knowing - and is one of the tougher things to track down and correct later). Spend some time to learn this skill - it is important. This subject is not to hard to understand - but is very hard to describe, using only words.