Satoshis are subunits

A satoshi (aka a “sat”) is a subunit of bitcoin. 1 satoshi is equal to 0.00000001 bitcoin (one-hundred-millionth of a bitcoin).

It’s kinda like how $1 US dollar can be divided into 100 pennies, except that with bitcoin it can be divided into 100 million satoshis. One bitcoin or 100 million sats, same thing.

What this means is that when you buy bitcoin, you don’t have to buy an entire bitcoin (100 million sats), you can just buy a fraction of bitcoin. As the price of bitcoin continues to rise, it’s become much more common for people’s first bitcoin purchase to be just a small fraction of a bitcoin. The Strike app lets you buy as little as $0.01 worth of bitcoin, enabling you to get into bitcoin in an amount that best suits your budget.

Satoshis are named in honor of Bitcoin’s pseudonymous creator: Satoshi Nakamoto. For convenience, people often use whole bitcoin units when referring to amounts greater than one bitcoin, and use satoshis (or sats) when referring to amounts much less than one bitcoin.

Satoshis are the smallest possible unit that can be denominated on the Bitcoin blockchain, according to Bitcoin’s rules. This means you can’t create a transaction in an amount less than one satoshi, making satoshis the indivisible unit of bitcoin.

A whole bitcoin or a piece of a bitcoin

The difference between a whole bitcoin and a part of a bitcoin is just a matter of scale. You can buy fractions of a Bitcoin (satoshis), a whole bitcoin (100 million satoshis) or multiple bitcoin (100’s of millions of satoshis). The idea of owning a whole bitcoin is really only a psychological threshold. A “wholecoiner” is someone who owns at least 1 bitcoin, but that’s just an arbitrary milestone, just like 100,000 sats, 1 million sats, or 10 million sats.

When buying, you can denominate in bitcoin terms, such as buying ₿0.001 bitcoin (aka 100,000 sats), or in cash terms, such as buying $100 USD worth of bitcoin.

The total possible supply of bitcoin is 21 million (₿21,000,000), as defined in the Bitcoin software. 21 million bitcoin is the same as saying 2.1 quadrillion satoshis (2,100,000,000,000,000).

What are millisatoshis?

Millisatoshis (aka millisats) are equal to 1/1000th of a satoshi (or a one-hundred-billionth of a bitcoin), and can be used to denominate transactions outside the Bitcoin blockchain, such as on the Lightning Network.

In the Lightning Network’s “off-chain” payment channels, transactions can be denominated in amounts as small as a millisatoshi, however, since the smallest amount that can exist on the Bitcoin blockchain is 1 satoshi, any sub-satoshi amount can never be settled on-chain. This means millisatoshis don’t exist as a real amount of bitcoin (an amount transactable on the Bitcoin blockchain), only as a tool for expressing micro amounts off-chain.

It’s kinda like how you could have a dollar-denominated contract for sub-penny amounts, such as a professional writer who earns $0.0955 per word. The writer can track her ongoing earnings, but when it comes to actually being paid, the amount will need to be rounded to a whole penny for the payment to be realized.

Subdividing bitcoin off-chain, such as millisats on Lightning, doesn’t increase the Bitcoin supply, it just takes the existing supply and denominates it in smaller units. It’s like how slicing a pizza into ever-thinner slices doesn’t create more pizza, it just takes the pizza you already have and cuts into tinier portions.

For global commerce, satoshis are likely a small enough denomination for pretty much all basic payment purposes.

TLDR; “stack sats” means “accumulate bitcoin”.