Hi Anton !
"Tie points" are actually matches between SIFT points (named "key points") detected on two (or more) different images.
They are the link between images to get 3D relative positioning !
Those points can be filtered, using different methods (Gradual selection tools in PhotoScan) in order to converge to a more precise relative orientation.
On picture overview panel, after the alignment step done, blue points are valid tie points used to get this 3D relative orientation, whereas grey points are disabled tie points, disabled because of their imprecision.
Most of the time, just a few points are directly disabled by PhotoScan, but you can get a better result using some well-employed filters.
Nevertheless, at first, trusting PhotoScan algorithms already give you great results if you import a good set of pictures
Regards