The boot loader is the program that allows you to select which operating system you want to use, and loads that operating system. You may have more than one boot loader installed, especially if you have more than one type of operating system installed. It is common for boot loaders to be able to load other boot loaders.
When resizing a partition, lots of data gets moved around. Many boot loaders don't understand the file system. They just remember where on the disk the required boot loader information lies. If this information is moved, they must be told where it has been moved. This is done by reinstalling the boot loader (i.e., re-running the installer program for the boot loader, which usually involves issuing a single command at the shell). Not all boot loaders require this.
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