You can train your crew at a cost per skill point and as in real life the higher your skill in certain area the better will perform in that activity and have many new options opened in-game. Crew members can also learn or improve skills on the job, this is cheaper since there is nothing to pay for but takes a lot longer to gain the same amount of skill. Each skill is capped at level 20 starting at 0, but there is a limit of skill points that a crew member can have depending on the traits and capacity.
In real life, a person can learn and be a master on one or two things but not a master on everthing or you can decide to have a Jack of all trades, master of none, but there is no limit on learning new things even if you are not a master on everything you can still learn new things and crew members will behave the same way as in human nature. So learning new things once reached the maximum number of given skill points assigned will make the crew member to lose mastery or knowledge on other skills that are low level. So this means that for example, if a crew member has a total of 25 skill points available, and is master in navigation with 20 skills points, and 5 skills points in weapons management, learning any new skill by using training will make the weapons management skill to go down by one point, so the total skill points at the end will still be 25 but allows players to re-skill or re-balance a crew member over the course of the game without having to start from scratch. If a crew member gains a skill point on the job while having all skill points assigned the new skill point gained will make the crew member to lose a skill point in another area so the total still remains the same.
Brain and learning capacity is limited so use it wisely.