pulley
Explanation
This comic is about the elaborate, paranoid measures a parent takes to deal with a child's fear of monsters under the bed.
In the first panel, a child asks "What are you building?" The parent explains they are building a "spring-loaded pulley system" so that later, when the child says there's a monster under the bed, the parent can reach under, grab hold, and pull the monster out. The child's response -- "Huh?" -- signals confusion.
The punchline comes in the final panel, where the child is in bed and says: "There's a monster under my bed." The parent, now positioned with the elaborate pulley contraption, appears ready to actually extract the supposed monster from beneath the bed.
The humor works because of the absurd overreaction. The normal parenting response to a child's fear of monsters is reassurance -- "There's no monster, go back to sleep." Instead, this parent has taken the child's fear completely literally and responded with an engineering solution. Rather than addressing the psychological fear, they've built a physical contraption to fight an imaginary threat.
This is a classic SMBC trope of taking something figurative or emotional and responding to it with excessive literal-mindedness and technical overkill. It also plays on the stereotype of the overly-engineering-minded parent who solves every problem with tools and mechanisms rather than emotional support.
The comic also advertises Zach Weinersmith's book "Sawyer Lee and the Quest to Just Stay Home," visible at the bottom of the strip.