Blockchain is available to everyone and everyone can see what Alice is doing with her bitcoin. Assume, she has 1 bitcoin and she sent it to Bob. We can check it in explorer since it is available to everyone. We also can trace what Bob is doing with his bitcoin received from Alice. Basically, there is a way to trace everyone's bitcoin address.
Provided that it is easy to trace bitcoin transaction, what will Alice do if she wants to send bitcoin to Bob's address without letting someone know? Bitcoin Mixer comes up with solution to execute untraceable transaction.
Simple process of sending bitcoin by Alice to Bob

Provided that Alice wanted to send bitcoin to Bob without a tail.
1. Alice will find out a Bitcoin Mixer.
2. Send her bitcoin to Bitcoin Mixer.
3. Bitcoin Mixer will send bitcoin to Bob.
If Alice send bitcoin to an address of Bitcoin Mixer, Bitcoin Mixer will not send bitcoin to Bob from that address. No one will know whom Alice sent bitcoin and from whom Bob receive the bitcoin.
The transaction will look like

For better understanding the system-
https://cryptalker.com/bitcoin-mixer/Some Mixing Services-
1. BitBlender
2. ChipMixer
3. Cryptomixer
4. Bitcoin Laundry
5. BestMixer
Before sending bitcoin to any mixing service, be sure the process, don't forget to check reviews from internet as well as don't forget to use TOR.
Am helping quote this whole post so that we would be able to see all the illustrations.
Alternatively, just use a privacy coin and skip all the work and fees associated with using bitcoin mixers. XMR is one of the best options considering the high trading volume and fairly stable price.
It really depends on the situation, Someone could just be in specific need of sending Bitcoins somewhere and not an altcoin.
For example, I would be in need of sending some bitcoins to my localbitcoins account, this renders using XMR or any other altcoin invalid.