In simple terms, this quote from Cervantes boils down to keep your mouth shut and pay attention, although Cervantes put it a little more elegantly than that.
The majority of people that I observe today, love to hear themselves talk. They never really listen to the people around them or pay any attention to what they are saying.
Their mind is always focused on the next witty thing that they can say to make themselves sound intelligent or humorous.
Rarely do these types of people learn anything from the people around them because they simply do not pay attention to the conversation.
They don’t engage their mind or put any thought into what is being said. As a consequence, they also tend to put their foot in their mouth on a frequent basis, saying things which either do not make sense or which are inappropriate, just to have something to say.
True being should use one's words sparingly.
One should spend more time listening and observing, than babbling away.
Using our words cautiously and thinking about what you are saying before you actually say it, are two traits which the warrior needs to develop.
These are traits of the wise being and the being of substance, and should be traits of the true warrior.
Always be wary about what you say and how you say it. Being careless with your speech can cause you many unnecessary problems.
Be slow of tongue, quick of eye, and quick of mind.
Namaste