schnallen
to get
Colloquial for to understand or grasp.
Ich habe es ihm dreimal erklärt, aber er hat es einfach nicht geschnallt.
I explained it to him three times, but he just didn't get it.

to get / to buckle
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to get
Colloquial for to understand or grasp.
Ich habe es ihm dreimal erklärt, aber er hat es einfach nicht geschnallt.
I explained it to him three times, but he just didn't get it.

The verb 'schnallen' (to buckle) is derived from the noun 'die Schnalle' (the buckle/clasp). This explains the literal meaning of fastening something with a buckle.
Der alte Koffer hatte noch Riemen, die man schnallen musste.
The old suitcase still had straps that had to be buckled.
You can recognize the colloquial meaning 'verstehen' (to understand) by the fact that the object is often abstract, e.g. 'es' (it), 'das' (that). The literal meaning 'befestigen' (to fasten) is used with concrete objects like 'Gurt' (belt/strap) or 'Riemen' (strap).
Nach deiner Erklärung habe ich es endlich geschnallt!
After your explanation, I finally got it!
The meaning 'verstehen' (to understand) or 'begreifen' (to grasp) is very colloquial. In formal or written contexts, you should avoid this word.
Komm, beeil dich, schnallst du es nicht? Wir müssen los!
Come on, hurry up, don't you get it? We have to go!
In its literal meaning, 'schnallen' (to buckle) is often used with objects that have a clasp or buckle. Typical phrases are 'den Gürtel schnallen' (to buckle the belt), 'den Helm schnallen' (to fasten the helmet), or 'die Riemen schnallen' (to buckle the straps).
Vor dem Start der Achterbahn muss man den Sicherheitsbügel festschnallen.
Before the roller coaster starts, you have to fasten the safety bar.
The idiom 'den Gürtel enger schnallen' (to tighten one's belt) is a fixed unit. It means that one has to save money, and the person who has to save is the subject of the sentence.
Wegen der hohen Inflation müssen viele Leute den Gürtel enger schnallen.
Because of high inflation, many people have to tighten their belts.
to understand
'verstehen' and 'schnallen' can both mean that you grasp something. 'verstehen' is neutral and fits almost any situation, including school, work, or formal conversations. 'schnallen' is colloquial and sounds casual, sometimes also a bit impatient or direct. You are more likely to say it among friends or in everyday situations. Learners may confuse the words because 'schnallen' is really similar in this meaning. But in polite or official situations, it is better to use 'verstehen'.

Jana schnallt erst nach ein paar Minuten, dass der Schlüssel noch zu Hause liegt.
Jana only gets after a few minutes that the key is still at home.
Sie versteht die Aufgabe, weil der Lehrer sie noch einmal erklärt.
She understands the task because the teacher explains it again.