Skill checks are a subsection of ability checks.

What are skill checks?

An skill or ability check tests a character’s or monster’s innate talent and training in an effort to overcome a challenge. The GM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure.

Do ability checks count as actions?

Some ability checks are actions, some aren’t. Many are, but it’s not a blanket rule. Sometimes a character does something as a part of their movement, which requires no action, and the DM decides (or the rules suggest) that it requires the character to make an ability check.

Skill checks are a massive part of DnD 5e, both in combat and outside of combat.