It is just like English sayings and phrases, there are Japanese sayings and phrases as well. It is called [kotowaza ことわざ].

Today’s kotowaza

勝って兜の緒を締めよ　katte kabuto no o wo shime yo

katsu – to win

kabuto – a samurai helmet

o – a string

kabuto

do not let your guard down after a victory, tighten the strings of your helmet after winning.

Its meaning

After winning a fight, tighten the strings of your helmet in order to brace yourself.

An example

Today’s basketball match was a easy win. Well, we will be able to match next one with confidence.

However, do tighten the strings of your helmet although you won. Let’s practice tomorrow as usual.

arigato