A sliding buckle retention system uses a simple buckle at the back of the helmet that you slide to tighten or loosen the fit around your head. It's the most basic adjustment type you'll find, typically on entry-level helmets under $60 or so. It works, but it lacks the fine-tuning precision that dial systems offer.
The main downside is that you can't easily make small adjustments while riding—one-handed on-the-fly tweaks are basically impossible. The fit also tends to be less even around your head compared to dial or yoke systems. If you're a casual rider sticking to smooth paths or short commutes, a sliding buckle can certainly get the job done.
However, if you ride longer distances, tackle varied terrain, or just want a more secure and comfortable fit, it's worth stepping up to a helmet with a dial system. The difference in everyday comfort is noticeable, and you won't be fighting headaches from an imprecise fit on longer rides.