The palmaris Longus tendon is a nifty wee evolutionary remnant that is absent 14% of the population.
You can find out whether you have this tendon by pressing your thumb & pinky finger together and looking at your wrist (which is also called a Thompson's Test). If you do have it, you'll see a noticeable tendon pop up - if so congrats, you have a Palmaris Longus 🎉
Apart from being a just OK-ish band name, the Palmaris Longus has no real significance and is thought to be a long forgotten tendon that we used to use to climb trees earlier in our evolution.
If you have this tendon the good news is that is in the off chance that you ever need some sort of wrist tendon graft, they can use this guy...
It doesn't affect grip strength in any noticeable way, so it's a purely pointless part of your body, right up there with our appendix, wisdom teeth & male nipples.
Useless facts from me to you. You're welcome.