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What is the rule of one?
The rule of one means that all Bethesda titles only ever use the last record of any given formid.  This is why we can change the game.

How it works is like this:  A formid is created.  Every succeeding record with the same formid then replaces that record in it's entirety.  So if I have a specific formid, and 100 people make changes, that means I now have 101 records with that formid.  They are then read sequentially by the game engine until the last record is read.  Every time the engine looks at the next record, it essentially forgets all the prior records and only remembers the last one read.  This means that 99 of those 100 changes are completely discarded and not seen by the game.  Since a patch is just another succeeding record, all patches are simply read in order and discarded if another succeeding record follows it.

Patches:  Patches take the base record and any changes to that record and create a new instance of that record with all the changes to date.
Lets take an NPC formid.  000A2C8E other wise known as HousecarlWhiterun "Lydia" [NPC_:000A2C8E].  In my load order, there are various mods that all edit different records of the NPC known as Lydia.  One edits weapons, another edits the NPCs looks, another adds a package, while yet another might edit her stats or perks or items.  If all of these different edits come from different mods, then without patches, only the very last edit to the NPC is used.  This means that all the other edits are lost.

So how does this work.  First Lydia is introduced in skyrim.esm where she is defined.  Next she is given a package and a script by USSEP, Then USMP changes some of her flags.  Then another mod (Sigaar Riften Carriage Driver Sandbox) also gives her some extra packages, while I gave her new armor with another esp.  My Breezehome mod next gives her yet another package to  redefine her sleep behavior, and Pandorable changes how she looks.  That's 6 changes by 6 different mods.  If we only use the last mod change, then Lydia looks how Pandorable made her look, but, she looses all the other changes because the skyrim engine only looks at the last record.  This is easy enough to fix by combining all the changes into a new record and putting it last.  However, unless you know
how to create patches, you are relying on different patches to do this for you and possibly a final patcher to amalgamate all the changes to a final record.
If you are simply relying on a variety of patches that allow two mods to work together and assembling them into a list without that final patch that amalgamates all the changes into a final record, then you are only getting results from the final patch.  This is the kicker.  All patches are actually replacement records.  They don't actually patch, they replace.

If we go back to Lydia, having a patch for USSEP, and another for USMP and yet another for every other mod will double the record count, but the last record is still the only one that is being used, and if it doesn't contain all the changes, then any change it does not have is lost.  So using multiple patches doesn't garantee results.  The only way to get all the changes is to use a patch that amalgamates all the changes into a single record that is then placed after all the proceeding records.  This can be created by hand or with a patcher such as Mator Smash, Wrye Bash or Synthesis.  That is how you get all the record changes because the rule of one simply discards all proceeding duplicated formids and uses the last one.  So effectively, my patch for lydia, patches all 6 esps at the same time.  How many mods are involved isn't important.  What is important, is that all the changes are carried over to that final record so they can show ingame.

While the example I used was an NPC, this applies to almost every record type in the various Bethesda games.  There are some exceptions to this that get amalgamated at runtime, (Navmesh is one), but with only a very few exceptions, all formids follow the rule of one.  It doesn't matter if its a cell or armor or magic effect or weapon or any other record type.  The rule of one always applies.