install-from-source refers to one method of installing software packages on Ubuntu without using the Software Center. It is generally not the recommended way of installing software.
When software is created, the code is written to
source files, which are then
compiled and installed. Most software packages are compiled by the developers before being distributed to the user, and all software downloaded and installed from the Software Centre has been precompiled for the convenience of the user. Compilation is the act of translating the
source code from the language in which it is written, which humans can understand, to the language which machines can understand.
Occasionally though, it may be necessary for the user to compile the
source code of an software package themselves. Usually it is only developers that do this so that they can test out modifications to the program before marking it as finished.
Hint: If you have to look up the definition of the word
compile in this context, you shouldn't attempt this.