You need to achieve 4 out of 5 stars to certify in a SKILLS module. However, you can replay modules as often as you like to do this! You're not expected to get everything right first time - this is all about learning by doing.
SKILLS modules with multiple items will have their own map. You may have to certify in a SKILLS module to unlock the next module. If you are unsure why a module is locked, refer to Why is a SKILLS module locked? for details. Generally, you have to certify in all prior modules in a journey before the next module is unlocked.
Some SKILLS modules will not have multiple items and instead of displaying the map, will directly open the game. Although they are not displayed on a map, they might still be locked and require you to complete modules prior to it in a journey in order for it to become available.
See also: Why is a SKILLS module locked? | Certification