Conditional enqueue is a WordPress coding habit of loading a script or stylesheet only on the pages that actually need it, instead of loading it everywhere by default. The phrase sounds technical, and it is, but the idea behind it is simple. Many plugins load all of their code on every page of your site, even pages that never use the plugin. A booking plugin might load its calendar code on your blog posts, your about page, and your contact form, none of which have a booking calendar on them. That extra code is dead weight that slows every page down. Loading it conditionally means the booking code loads on the booking page and nowhere else. This is one of the most common things I fix when a WordPress site is slow: a handful of plugins each insisting on loading everywhere, adding up to a site that drags. A site owner will never write this code themselves, but the term is worth knowing, because "the plugin isn't conditionally enqueueing its assets" is a real, fixable diagnosis rather than a shrug.
Glossary entry