Period
Period
[Cueball is standing with a hand to his chin looking towards a woman off-panel right. In all panels the woman answers off-panel with a small starburst close to the right panels border indicating from where her speak emanates.] Woman's voice (off-panel): Ugh. Stupid uterus. Cueball: Hey, your period is every 28 days, right?
[Cueball is standing hands down.] Woman's voice (off-panel): Yes, why? Cueball: Well, period = T = 1/f. Woman's voice (off-panel): So?
[Cueball holds up a finger in front of him.] Cueball: Using this, we can calculate something you already know. Woman's voice (off-panel): What?
[Cueball is standing hands down.] Cueball: Your uterus-hertz. Woman's voice (off-panel): If I could get up I'd smack you.

A period, also called a menstrual cycle , is a process marked by (among other things) a few days of abdominal cramps occurring roughly once a month for women in the reproductive age .
"Period" ( T ) is also the term in mathematics for a measure of the rate at which something happens. It measures the length of time between occurrences. For example, a full moon happens about once every 29.5 days, so its period is 29.5 days. By taking the inverse of this, we can get a different measurement, its frequency ( f ) - the number of times an event happens within a given length of time. This is usually measured in hertz (pronounced the same as "hurts"), which is the number of times something happens in a second. Hertz is abbreviated as Hz, and since f = 1/T, with T measured in seconds (s), it follows that 1 Hz = 1 s −1 .
The woman has her period every 28 days. Since we can calculate the frequency based on the period, we can, indeed, calculate her "uterus-hertz" but she already knew that her "uterus hurts". Yes, this has all been a setup to a terrible pun to make your head hertz. And if she had not been in such terrible pains that she cannot even get up, she would have smacked Cueball for it, which would really hertz. This makes it even more hertzfull by Cueball, to make fun of his girlfriend in such a state.
The woman off screen is probably Megan given that she is most often depicted as Cueball's girlfriend, and the fact that he is in her home with her on the couch is a good indicator that they are intimate. (This is not enough to include this comic as one with Megan).
The title text gives the other answer that can be calculated from Cueballs musings, the actual frequency of the womans  period which have a period T = 28 days. 413 nanohertz is the correct frequency as shown here by Google Calculator.
It can also easily be shown here:
f = 1/28 days = 1/(28 days × 24 hours/day × 60 minutes/hour × 60 seconds/minute) = 1/(2,419,200 s) = 4.1336 × 10 −7 s −1 = 413 × 10 −9 Hz = 413 nHz .
[Cueball is standing with a hand to his chin looking towards a woman off-panel right. In all panels the woman answers off-panel with a small starburst close to the right panels border indicating from where her speak emanates.] Woman's voice (off-panel): Ugh. Stupid uterus. Cueball: Hey, your period is every 28 days, right?
[Cueball is standing hands down.] Woman's voice (off-panel): Yes, why? Cueball: Well, period = T = 1/f. Woman's voice (off-panel): So?
[Cueball holds up a finger in front of him.] Cueball: Using this, we can calculate something you already know. Woman's voice (off-panel): What?
[Cueball is standing hands down.] Cueball: Your uterus-hertz. Woman's voice (off-panel): If I could get up I'd smack you.
