This popup shows at random times. I want to know if it is legit or not.
It's from Verizon Update Center saying that the "In-Home Agent (IHA) is not running at top speed and wants me to run Verizon's Optimizer Tool on my PC.
However if you have a current operating system, seriously consider ignoring the Verizon Optimizer. Your operating system will probably set better settings. Run a speedtest to see if your speed is good or not.