It just means that on a given server, the DLL and EXE files that make up the behavior of SQL Server can, for the most part, be installed once, and then used between multiple instances on that server. You can have, for example, in-memory OLTP on two instances. Same goes with the other functionality.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning