Cookies ! Browsers war against it.

Dive into your web browser’s settings and you’ll see references to cookies—little bits of data that have been around almost as long as browser apps themselves. Now, browsers are going to war against them, and it’s going to affect you.

Earlier this year, Google announced it wants to block certain cookies on Chrome, but it’s not the first—Mozilla and Apple have spent years tightening the rules around how Firefox and Safari, respectively, handle cookies.

If you’re wondering why this is happening, or even what cookies are in the first place, let’s start with a basic definition: Cookies are small files that websites leave on your computer, phone, tablet, and other digital devices whenever you visit. When you load up a website, the page checks to see if it’s ever left a cookie with you.

That’s useful for a variety of reasons. For one, it tells the site whether you’ve visited before, so you might not get the same “subscribe to our newsletter” or “claim this discount code” alert you did when you landed there for the first time.

Cookies can also hold data about your location, so you’ll see news reports or weather forecasts centered on your area without having to specifically tell the site where you live every time you open it up. They may further help out by saving the details of how you’ve set up a site’s layout so it looks exactly how you left it.

If you’ve provided a site with personal details such as your email address and phone number, those might be stored in cookies, too, along with details about what you’ve looked at in the past on a particular webpage.

This might be ringing some privacy alarm bells in your head, but cookies are really just local copies of information these sites have anyway. You can’t use Facebook without providing your email address, and Amazon wouldn’t work well if the site couldn’t see the products you’re looking at—cookies are just an extension of all that.

At their best, cookies should make your online life easier and more convenient, but if you want to see what navigating the web is like without them, try opening a private or incognito browser window. Doing so will block access to any cookies currently on your device, so sites won’t know who you are and nothing will be personalized. You’ll also need to log in everywhere you go. Using the web without cookies is a bit like going online on a brand new computer.

But cookies can be used to build a picture of who you are and what you do online, which is why browsers are tightening the rules about what they’re allowed to do. Under particular threat are third-party cookies—these are usually embedded in advertisements and can track you across multiple sites, not just one.

Ordinarily, a site can only access the cookies it has created itself—Facebook can’t look at your Amazon cookies, for example. However, third-party cookies (often served up by advertisers or data brokers) can do some cross-site tracking. That means marketing organizations might be able to connect the dots between, say, your online shopping habits and your social media activities.

Safari and Firefox block third-party cookies by default, but you can choose to allow them if you want. For now, at least, it’s the opposite in Chrome: The browser automatically allows third-party cookies, though you can choose to block them in the browser settings. It’s not yet clear how Google’s cookie-blocking plan will affect this, if at all, but you may see some changes down the road.