When someone clicks an ad, the platform often adds a unique identifier to the landing URL: gclid for Google Ads (with gbraid and wbraid in some privacy-restricted iOS cases), fbclid for Meta, ttclid for TikTok, msclkid for Microsoft Ads, and others for LinkedIn, Snapchat, Pinterest, Reddit and X.
Click IDs serve two purposes. They identify the visit as coming from that platform, and they let you send a conversion back to the platform tied to the exact click, which is how offline conversion imports and conversions APIs match sales to ads. That's why capturing the click ID on landing, and storing it with the person's journey, matters even when the sale happens weeks later.