All day, you breathe air in and out. This is to get yourself oxygen, a type of atom which is very important to have. Your body needs oxygen because of one thing oxygen has, that not many other atoms have. Oxygen is very sticky. It's one of the stickiest atoms there are. Oxygen likes to stick to stuff because of it's "electronegativity", and it will try to stick to whatever it can. When you breathe in oxygen from the air, it comes in pairs. These are two oxygen atoms stuck together, making their stickyness cancel out. Your body knows how to unstick them and use them to glue other atoms together to form molecules that can be used throughout the body to make things, from bones to skin. But when a single lonely oxygen atom gets free, we call it a "free radical" A free radical is just a single oxygen atom, but remember how we said oxygen is very sticky? It will try to stick itself onto other molecules, even when it shouldn't! This can easily break things, because the molecules in your body have to be very carefully made. If an oxygen molecule just walks up and sticks to it, this is very bad. Lucky for us, there's other types of molecules called "antioxidants". Antioxidants are molecules too, but they have a special slot just for sticky oxygen atoms. Antioxidant molecules are sticky too, but they have a shape that only fits with a sticky oxygen atom. If you eat antioxidants, they will go around your body gobbling up all the sticky oxygen atoms, before they stick to something they shouldn't. They only work on sticky oxygen, they don't work on the paired oxygen you breath in, so don't worry about them using all the oxygen in your body. Some foods just happen to have a lot of antioxidants. It's because food is made up of molecules too, and some of those have more antioxidants than others. They make them on their own as they grow so they don't get broken by sticky lone oxygen molecules either. Your body makes it's own antioxidants too! But sometimes it can't make enough on it's own, so you need to eat them to get more. The healthier you eat, the more antioxidants you can get, and the less likely your body will get hurt by those sticky lone oxygen molecules. More on reddit.com

Chemically speaking, a radical is an atom which has a "free" electron, one that is not bound and not an electron pair. A good example is the oxygen radical O2(-), the hyperoxid anion. These molecules have a strong urge to react with stuff - in the lab and in your body. Your body usually deals with them very well, you have enzymes which are super fast in converting radicals into less toxic species. But these enzymes have limits. If you have too much free radicals, they will form DNA-adducts, damage your cells or lead to inflammation (this is called oxidative stress). Marketing techniques nowadays often tell you that this or that acts as a radical annihilator and it is healthy for you. That may be true on a molecular level, but the effect is so tiny that often you don't see it. Your body is usually capable of dealing with "normal" stress. Fun fact: A famous story about Pauling was that he ate a spoonful of ascorbic acid (which binds radicals) every day to protect against cancer and flu. He died with 93 years. More on reddit.com

Essentially, free radicals are highly reactive and have a tendency to damage biological things. But sometimes you NEED to damage biological things, and in those cases it's good to have some around. Ultimately, you've evolved to have a certain level of free radicals, and any deviation from that level will make your body behave in a manner it isn't optimised for. That means that too many or too few are both potentially dangerous for different reasons. More on reddit.com

Free radicals inside your body are a source of damage/oxidative stress.

Putting baking soda in your bath water won't do anything for that, though.

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