Skin irritation due to continuous friction. Loose fitting clothes & shoes, non-technical fabrics & moisture increase the chances of suffering them. Your skin is also a big factor, with each individual being less (harder skins) or more (softer/”untrained”) prone to chafing.
Main ways to prevent them:
Lubricants: Once upon a time we used to use vaseline on armpits and inner thighs for long distance swimming. Now there are better, more sport specific, products out there. The best known might be Body Glide, but there are several other brands in the market.
A deodorant-like stick to be applied on the friction areas before getting dressed. Depending on the activity these will be different. For swimming, mentioned armpits & thighs, also neck and others if using a wet-suit. For hiking, on your feet and if you are going to be wet for long (stream hiking, hot humid days) thighs again, arm(pit)s in the contact points with backpack straps, etc. For running guys, nipples! Seriously. I have told this several times to friends who ignored the advice and ended with serious chafing (surprise burning in the post event shower) or even bleeding… Paraphrasing Kilian Jornet “you end up putting that stuff everywhere but your hair”.
There are gel or powder (mainly to keep skin dry) version products available too.
Technical clothing: Avoid cotton. It holds moisture and does not allow your body to regulate its temperature. You should be looking for seamless, breathable, lightweight materials that allow moving as much moisture away from the skin as possible. Usually polyester, nylon, lycra, etc.
Socks technology has improved greatly. From simple Nike dri fit Right/Left specific socks to Drymax or others. In my case toe sock are not so a good option (foot structure and skin), but quite some trail ultra runners love Injinji and other similar brands.
I like my Drymax though. Two layered socks that remove the moisture from the skin. I use them for any run longer than 90 minutes, long hikes, stream hikes…
Expensive but completely worthwhile for me, who tended to have tons of blisters. By the way, if you buy them, DO NOT use Body Glide or similar products too on your feet. And they are not miraculous. If you have too fit or loose or hard shoes finally you will get blisters on.
Try different options and see which one works for you.
Treating blisters: If you have them the best solution would be to rest the area and try to avoid breaking the skin. Using flip flops instead of shoes for example.
If you need to continue hiking/running next day, you have different options. There are film dressing available in pharmacies or trail running shops: Compeed & Second skin come to mind. Dry carefully the area and cover the blister with the correct size transparent film. It will work like a second skin, holding even for several days.
In case of emergency you can try the old fashion boy scout blister draining method (doctors don’t read further). “Sterilize” a needle (with alcohol), carefully punctuate the blister and try to remove all the liquid out. If you have a clean thin thread you can, carefully again, pass the needle through the blister, leave a little piece of threat in. Remove as much liquid as possible without breaking the skin. Tight the thread. Overnight the thread will absorb the remaining liquid, cut & remove it and luckily you will be able to walk further without breaking the skin. I have used this technique before and worked well, but here explained its dangers.