When you have cleaned or disassembled your Ultimaker 2+ printhead, ideally you want to put it back together fully calibrated. This spacer can help you determine the right space in your printhead, by placing it between the aluminum brackets, and tighten the 4 thumbscrews until this spacer is stuck in between.

There are 2 important guidelines to keep in mind..
1: The middle part MUST be 16,8mm. If it is bigger or smaller, the spacer has lost its function. Therefor, do not scale the model and measure it before you apply it to your printhead.
2: Do NOT tighten the printhead too tight trying to grasp this spacer. With most materials, but especially PLA, too much tension on the thumbscrews is more likely to deform the spacer than help you find the correct calibration. Tighter is not better. 16,8mm as a gap in your printhead is what you are after.

Good luck!