The detector upgrades
The CMS tracker is the charting process that follows how a particle moves through a magnetic field. The inner pixel detector and an outer strip detector will be completely replaced. The tracker is the part of the CMS detector closest to where LHC proton particles collide, in the heart of the detector. The main reason the upgrades are needed in this innermost area is that the HL-LHC will collide with protons much quicker, and the proton paths will rapidly run into each other and pile up.
One of the newest additions to the detector is the pixel detector. The pixel detector has a finer granularity. Therefore, the rates must be higher, as well as the granularity. This is because the particle paths are happening faster, and the increases in granularity and rates will help to actually detect individual particles. Otherwise, the particles go through the detector so fast that the resulting particle images are just smears.
There are several other layers of detectors. The timing detectors are meant to give precise times to the motion along particle paths. The CMS trigger and data acquisition selects the most potentially interesting collision events and then records the relevant data. It discards the more benign events to keep the huge volumes of imagery information manageable.