The difference between Buckle down and Buckle up
When used as verbs, buckle down means to put forth the needed effort, whereas buckle up means to fasten one's seat belt or safety belt.
check bellow for the other definitions of Buckle down and Buckle up
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Buckle down as a verb (idiomatic):
To put forth the needed effort; to focus; become serious; apply oneself (e.g. to work or study).
Examples:
"If he would buckle down and do his homework, he could be an excellent student."
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Buckle up as a verb (intransitive, idiomatic):
To fasten one's seat belt or safety belt.
Examples:
"Buckle up every time you drive somewhere in a car, and make sure your passengers buckle up, too."