Players have 5 trainable skills. These are the skills that can be improved using experience points. The are typically low for young players. These are by far the most important measurement of the strength of a player.
Main skill for: goalkeepers
The blocking skill is used to block shots and gain posession of the ball. For goalkeepers this is the decisive skill because it determines the chance he can stop a shot on goal
Other players can also block shots on goal when they are in the penalty area but blocking is less effective for them. They also use blocking to intercept passes
Main skill for: defenders
Dueling is used to capture the ball from an opponent. When a player is in the same region as an opponent with the ball, he can attempt to capture the ball. The dueling skill is the most important factor in determining the success.
Dueling also helps to defend when you have the ball, but it's less effective. It's also very important in corner and free kick situations where a lot of players are close to each other and fight for control of the ball
Main skill for: midfielders
Passing is used for passing the ball from one player to one of his teammates. If two players pass the ball between each other, the highest passing skill of both players determines the chance on success. This makes this skill important for midfielders because they can connect your defenders with your forwards. High passing skills means your players can make longer passes.
Passing is also used for giving crosses to your forwards. If they immediately shoot on goal after receiving a pass it's much easier to pass the defenders and keeper than if they'd try to shoot from a situation in which they already had the ball. High passing skill of the player giving the assist means there is a bigger chance the pass actually reaches the forward and also decreases the chance he'll miss the target completely.
Finally, passing is used for the player taking free kicks and corners. High passing increases the chance that something good will come out of a free kick or corner.
A player who plays as goalkeeper gets a passing bonus
Main skill for: forwards
Scoring is used to score goals. High scoring increases the chance to pass the keeper and defenders.
A player playing as defender gets a scoring bonus
Secondary skill for: all players
Tactics is a special skill. It does not directly make your players stronger but it makes them take smarter decisions.
A player with high tactics moves later and acts sooner than a player with low tactics. This means he can move after other players have already revealed where they went and can shoot, pass or tackle before anyone else can.
Of course, this doesn't help much if they then make a mistake. It also doesn't help to have tactics twice as high as your opponents, it just needs to be higher. Take a look at your strongest league opponent to see how much tactics their players have.