This is the 14981st most frequent German word.
schnallt
"Grasps" or "understands" (informally), or "straps" (depending on context).
In this context, 'schnallt' refers to the act of fastening or tightening something, specific to a belt in this case.
Er schnallt den Gürtel enger.
He tightens the belt.
Here, 'schnallt' is used colloquially meaning 'to grasp' or 'to understand' a concept or information.
Sie schnallt endlich die Mathematikaufgabe.
She finally understands the math problem.
In this usage, 'schnallt' relates to secure or fasten something, namely a backpack, and indicates effortless action.