Multiple Attack Penalty

The more attacks you make beyond your first in a single turn, the less accurate you become, represented by the multiple attack penalty. The second time you use an attack action during your turn, you take a –5 penalty to your check. The third time you attack, and on any subsequent attacks, you take a –10 penalty to your check. Every check that has the attack trait counts toward your multiple attack penalty, including Strikes, spell attack rolls, certain skill actions like Shove, and many others.
Some weapons and abilities reduce multiple attack penalties, such as agile weapons, which reduce these penalties to –4 on the second attack or –8 on further attacks.

Always calculate your multiple attack penalty based on the weapon you're using on that attack, not ones you used on previous attacks. For example, let's say you're wielding a plasma sword in one hand and a battleglove (which has the agile trait) in your other hand, and you make three Strikes with these weapons over the course of your turn. The first Strike you make during your turn has no penalty, no matter what weapon you're using. The second Strike will take either a –5 penalty if you use the plasma sword or a –4 penalty if you use the battleglove. Your third attack would be a –10 penalty with the plasma sword and a –8 penalty with the battleglove, no matter what weapon you used for your previous Strikes.
The multiple attack penalty applies only during your turn, so you don't have to keep track of it if you can perform a Reactive Strike or a similar reaction that lets you make a Strike on someone else's turn.