"Lateral only" means the toe piece releases sideways to protect you in twisting falls, but it won't release upward. So if you take a fall that forces your toe up—like catching an edge and pitching forward—the toe piece stays locked in place.
This release style is common in entry-level bindings and older designs. It handles the most common fall type (twisting) just fine, but it leaves a gap in protection for forward falls where upward force is involved.
For most modern skiers, bindings with lateral + upward release are the standard and offer better overall protection. If you're skiing aggressively, in the park, or in variable conditions, the extra upward release can make a real safety difference. Lateral-only bindings are best suited for casual, groomed-run skiing at moderate speeds where forward falls are less likely.