Sodium

I keep sodium on the low side due to some health issues it exacerbates. You have to read labels - it hides everywhere. (I just turned down some tortellini my husband made - not due to the carbs, but due to the sodium. 700mg per 1/2 cup dry!) Anything processed (canned, packaged) is going to be high in sodium unless it's sold as a "low sodium" product. But it lurks where you don't expect it, like packaged breads for example. As nutmegoreo said, look through your diary, see where it's coming from, and find alternatives. More fresh fruit and veggies and less convenience food, bottled sauces and dressings, etc.

Higher sodium intake can cause you to retain more water, which will mask weight loss, but it won't stop you from losing. Eating at restaurants/fast food will also cause your numbers to go up. Ultimately, though, if you don't have a medical reason to watch your sodium (or sugar) intake it isn't a big deal. I don't even track them.

I'm on a low sodium diet because my doc wants me to watch my blood pressure. Staying away from processed foods, as hard as it is, pretty much does the work for me. (I have my sodium set to 1300). I don't use salt when I cook, except in baked goods that need it for leavening.
Beware of things like tamari and soy sauce, ketchup, and hot sauces because even the 'low sodium' varieties are sodium-packed. I have a huge variety of those salt-free seasonings that all have different flavors and keep food from getting boring. Safeway makes a great 'all purpose' as does McCormicks, and McCormick's makes a 'spicy Southwest' that is great for making soups and chili.

But basically what malibu927 says- If you don't need to watch your sodium for medical reasons, it's really not a big deal unless you're gobbling down a pound of pepperoni dipped in soy sauce every day or something.

Sodium is an unsexy mineral that gets attention because there's a lot of heart patients out there. Some kidney patients too, but mostly heart. It's used for the body's electrical signals and to maintain the amount of water in cells. More than enough is more for the kidneys to do and less than enough can land people in the emergency room. Enough is different for each person and depends on things like the weather they're in, and sometimes enough falls outside the suggested limits. Don't worry about it much unless -your- doctor says to or you're running into problems.

I watch my salt intake, too, just because an overload makes me feel like crap.

As others have said, you have to read the labels on packaged food. Soups, especially, can be crazy high in salt. Buy low or no sodium tomato sauces and jazz them up with fresh vegetables and herbs to dress pasta. It's pretty easy to make salsa from it, too.

Salt-free potato chips are my new go-to snack... a lot of people have told me they are horrified that I can eat them, but they simply taste like potatoes and nothing more. Dipped in mustard or cider vinegar, they're delicious.

If you don't have the 'sodium' field activated in MFP you might want to show it so you can see. Even whole foods have naturally occurring salt content that might be surprising.