"Logic probe is small digital instrument like a pen, used to check the logic (Hi or Lo) at the different stages in any digital board."

It usually has a sharp metal point, so that you can make a good electrical contact at the connection you're investigating.

"Hi means 5V and Lo means 0V according to the TTL levels."

Actually, it's more complicated than that; the electronics within the probe are carefully designed so that it will only give a 'Hi' indication on a signal which definitely meets the requirements of a 'Hi'.
Similarly for 'Lo', and some probes can also give an indication when a signal is in the "udefined" area between the two.

Some probes are switchable for different logic types - TTL, CMOS, etc.

Some probes also give an indication of when a signal is pulsing.