​URL parameters, also known as query parameters or query strings, are components of a URL that are used to pass data to a web page or web application. They appear as key-value pairs separated by ampersands (&) and are typically included in the URL after a question mark (?).

Let's use a short example in order to understand this better:
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Imagine you have a page, with a contact form on it. This form only has 2 fields, a first name field, and a last name field.

This is the URL of that form:
​https://exampleform.onepage.me/
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If we want to add data to the first name field, we can use this modified URL:
​https://exampleform.onepage.me/?fname=John
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To start a URL-Parameter, we use a question mark (?).
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Then we specify what field should be filled out by the Parameter, in this case "fname".
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After the field, we add a (=).
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Finally, we finish the URL-Paramter with the actual data, in this case, "John"
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​Note: if you want to address another field you can add an ampersand "&" and start with another field. Example: https://exampleform.onepage.me/?fname=John&lname=Doe

UTM parameters are five variants of URL parameters used by marketers to track the effectiveness of online marketing campaigns across traffic sources and publishing media.
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These 5 Paramters are:
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utm_source
utm_medium
utm_campaign
utm_term
utm_content
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When you use your page URL in ads, Meta and Google add these parameters automatically. These do not require actual visible fields in the form. They are added to every form in the background.
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When a lead has filled out your form, the URL Parameters can be found in the bottom right of the lead info:
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Hidden fields are a special kind of field that you can add to any contact form like this:
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Contrary to any other field, these are completely invisible on your page and can only be filled by using URL parameters.
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When you click on a hidden field, it comes with 3 options:
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Label: This is the internal name of your hidden field. This will only matter in your project's CRM. It will be the name used here:
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Variable name: This is the name that will be used to address this field in the URL.
If you use "hidden_field" here, you need to add this:
​https://exampleform.onepage.me/?hiddenfield=Example
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to your URL.
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​Default value: If somebody manages to delete the URL-Parameter or opens your page via the direct link without the URL-Parameter, the default value will be used.

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