Title text: And the worst part is you won't apologize.
A diode is an electronic component that, to put it simply, blocks current traveling in one direction, but allows current going the other way. Diodes are a common component of digital circuits, such as those found in a computer (in particular, transistors, the fundamental building blocks of modern computers, are essentially two diodes stuck together). The end that rejects incoming current is marked with a line or band of paint.
The comic is describing a common frustration in relationships. Sometimes one party in a relationship will feel that the other isn't listening to them, even though they themselves are being as open as possible. This is analogous to the function of a diode, which the comic indicates by literally introducing a diode into the dialogue. The diode introduced has the banded end being held by Cueball. This means his "negative energy" is blocked by the diode, while Megan's flows through to him. This means she never receives his emotional pain, so does not feel she did anything wrong. The title text simply further illustrates the point: Megan didn't perceive Cueball's apology, due to it being blocked by the diode, so she berates Cueball for not making excuses.
- [Cueball is talking to Megan. Megan holds up a diode.]
- Cueball: We need to talk.
- Megan: Okay, but first hold the end of this diode.
- [They hold the diode.]
- Cueball: You hurt my feelings yesterday.
- Megan: You embarrassed me with my family last weekend.
- [They are still holding the diode between them.]
- Cueball: I'm sorry.