I am getting more Queries from SharePoint community about the below, so explaining in this article.
Definition: when we Install the CU in the farm, we see the Installation required and upgrade blocked in the status of servers under the system settings in CA.
Installation Required : The server still not yet install with the same CU that already installed in other servers.
Upgrade Blocked : The server is blocked from upgrade. current Server is installed with latest CU and it will remained blocked until all the SP servers in the farm are installed with the same version.
Hope it helps 😛