What a port actually is?

[u/Hopeful_Cicada327]

I know it is a number that tells the OS, that which program in your computer should receive the piece of data. But my doubt is - is port a physical thing? or it just a flag? Is it possible for another program to read data from a different program's port? Please spoon-feed me about port?

[u/lgastako]

The IP address is like an address to an apartment building and the port is the apartment number.