camelCase joins words with no separators and capitalises each word after the first: pageLoadTime, metaDescription, totalClicks.
Where it appears
JavaScript variables and object properties, JSON keys in most APIs, and analytics event and parameter names in many platforms.
Why the convention matters
Systems that expect camelCase generally do not accept anything else. A key sent as pageloadtime to an API expecting pageLoadTime is silently ignored rather than rejected, which is considerably harder to debug than an error.
Converting in bulk
A list of human-readable labels becomes a list of valid keys in one pass, which is the usual need when specifying events or fields from a planning document.
Try it: Convert to camelCase on SeoWolf's Notepad.