1.
Rate something lower than its real/actual value , under appreciate.
2.Estimate/
rate someone lower than his/her actual potential, qualifications.
You underestimated that problem because you are not familiar with it.
Don'
t underestimate 'em simply because they are young.
Julian: Ask your lil bro to quit that chalenge it is very hard for him.
John: Don'
t underestimate my little brother his able.