Assuming Alice and Bob wants to make use of lightning network, Bod is the owner of one coffee shop, Alice is the customer. Both of them will have to open a channel, the channel is just a 2-of-2 multisig, which means both of them will have to initiate the transfer of any particular fund.

If they both agree to put 0.005 btc each on the channel

Alice: 0.005 btc
Bod: 0.005 btc

That means they both have 0.005 btc + 0.005 btc = 0.01 btc on the channel. Assuming Alice went to Bob's shop to seeing coffee and he paid 0.00005 btc, the money will be deducted from Alice btc to Bob Btc on the channel. Which means Alice will have 0.0049 btc as 0.0001 btc is reduced by her on the channel while Bob will have 0.0051 btc for the coffee Alice paid for and so on until anyone of them closes the channel.

You can try use Electrum wallet and use very low amount of Bitcoin with your friend to understand this better using this guide.

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Nite that on-chain miners fee will be demanded for while opening and closing a channel, but you can open and close channel when fee is low. Lightning network fee is very low after you have open a channel alredy.