Like the other posters said above it can be as easy as just a simple gradient. You can kind of see that the gradient is based on the colors of the photo behind it to try to make it look like a subtle transparency with a copy of that part of the image behind it blurred.
You could do the same thing in illustrator but it would be easier to do this in Photoshop by stacking a second photo on top of the other. Between the two photos make a white bar the size of the bar you need. Blur the photo with Gaussian Blur use the blurred piece and the bar and make a clipping mask so that you now have what looks like a blurred bar of the image below and you should be able to play with the transparency of the blurred image and the bar to get close.
Photshops gradient options are also easier to use than Illustrators so you could probably get that effect with Photoshop much quicker if you wanted it to be an exact replica of what is in the pic you posted.